[PATCH v2 1/2] lib: sbi: Wait in sbi_hsm_hart_wait() till the hart is prepared to start
Anup Patel
apatel at ventanamicro.com
Fri Mar 3 08:02:53 PST 2023
On Fri, Mar 3, 2023 at 8:31 PM Evgenii Shatokhin <e.shatokhin at yadro.com> wrote:
>
> On 02.03.2023 07:07, Anup Patel wrote:
> > On Tue, Feb 28, 2023 at 9:26 PM Evgenii Shatokhin <e.shatokhin at yadro.com> wrote:
> >>
> >> On 28.02.2023 08:06, Anup Patel wrote:
> >>> On Wed, Feb 22, 2023 at 2:39 AM Evgenii Shatokhin <e.shatokhin at yadro.com> wrote:
> >>>>
> >>>> "wfi" instruction is not guaranteed to wait for an interrupt to be received
> >>>> by the hart, it is just a hint for the CPU. According to RISC-V Privileged
> >>>> Architectures spec. v20211203, even an implementation of "wfi" as "nop" is
> >>>> legal.
> >>>>
> >>>> As a result, a secondary (non-boot) hart waiting in the while loop in
> >>>> sbi_hsm_hart_wait() might leave the loop and continue even if it got no
> >>>> IPI or it got an IPI unrelated to sbi_hsm_hart_start(). This could lead to
> >>>> the following race condition when booting Linux, for example:
> >>>>
> >>>> Boot hart (#0) Secondary hart (#1)
> >>>> runs Linux startup code waits in sbi_hsm_hart_wait()
> >>>>
> >>>> sbi_ecall(SBI_EXT_HSM,
> >>>> SBI_EXT_HSM_HART_START,
> >>>> ...)
> >>>> enters sbi_hsm_hart_start()
> >>>> sets state of hart #1 to START_PENDING
> >>>> leaves sbi_hsm_hart_wait()
> >>>> runs to the end of init_warmboot()
> >>>> returns to scratch->next_addr
> >>>> (next_addr can be garbage here)
> >>>>
> >>>> sets next_addr, etc. for hart #1
> >>>> (no good: hart #1 has already left)
> >>>>
> >>>> sends IPI to hart #1
> >>>> (no good either)
> >>>>
> >>>> If this happens, the secondary hart jumps to a wrong next_addr at the end
> >>>> of init_warmboot(), which leads to a system hang or crash.
> >>>>
> >>>> To reproduce the issue more reliably, one could add a delay in
> >>>> sbi_hsm_hart_start() after setting the hart's state but before sending
> >>>> IPI to that hart:
> >>>>
> >>>> hstate = atomic_cmpxchg(&hdata->state, SBI_HSM_STATE_STOPPED,
> >>>> SBI_HSM_STATE_START_PENDING);
> >>>> ...
> >>>> + sbi_timer_mdelay(10);
> >>>> init_count = sbi_init_count(hartid);
> >>>> rscratch->next_arg1 = arg1;
> >>>> rscratch->next_addr = saddr;
> >>>>
> >>>> The issue can be reproduced, for example, in a QEMU VM with '-machine virt'
> >>>> and 2 or more CPUs, with Linux as the guest OS.
> >>>
> >>> Good catch. This is a valid issue which needs to be fixed.
> >>>
> >>>>
> >>>> This patch adds an auxiliary per-hart state variable, aux_state, as well as
> >>>> its accessor functions, to synchronize execution of the boot hart and the
> >>>> starting secondary hart.
> >>>>
> >>>> Initially, aux_state is 0: sbi_scratch_alloc_offset() zeroes the allocated
> >>>> memory. The boot hart sets it to 1 after it has set next_addr, etc., for the
> >>>> secondary hart, then sends the IPI to the latter.
> >>>>
> >>>> The secondary hart waits in sbi_hsm_hart_wait() both for aux_state to become
> >>>> non-zero and for an IPI to happen. This should be safe even if the secondary
> >>>> hart somehow gets an IPI unrelated to sbi_hsm_hart_start().
> >>>>
> >>>> Memory barriers are used to make sure the writes to next_addr, etc., in the
> >>>> boot hart are visible to the secondary hart if it reads non-zero aux_state.
> >>>>
> >>>> Only one bit of aux_state is actually used here, so, this per-hart variable
> >>>> can be used to hold other data in the future, if needed.
> >>>
> >>> Adding aux_state is an overly complicated solution in my opinion.
> >>>
> >>> Instead, you can simply do the following:
> >>>
> >>> rscratch->next_arg1 = arg1;
> >>> rscratch->next_addr = saddr;
> >>> rscratch->next_mode = smode;
> >>>
> >>> /*
> >>> * Barrier to ensure that the above writes have happened before
> >>> * updating the HART state.
> >>> */
> >>> smp_wmb();
> >>>
> >>> Before "hdata = sbi_scratch_offset_ptr(rscratch, hart_data_offset);" in
> >>> sbi_hsm_hart_start().
> >>
> >> True, the proposed fix it is more complicated than I would like it to
> >> be, but there is a reason.
> >>
> >> Should we worry about the scenario when two or more harts enter
> >> sbi_hsm_hart_start() with the same target hartid? It must not happen
> >> during normal operation of the OS, but, in theory, errors in the
> >> higher-level software could lead to this.
> >>
> >> Setting hstate to SBI_HSM_STATE_START_PENDING atomically and checking
> >> its previous values before changing *rscratch in sbi_hsm_hart_start()
> >> guarantees that the current hart has exclusive access to *rscratch after
> >> that. If we move rscratch->next_arg1 = arg1, etc., before hdata =
> >> sbi_scratch_offset_ptr(...), it would be possible for two harts to race
> >> and write different values to *rscratch for the same target hart. It is
> >> very unlikely to happen, but still.
> >>
> >> If it is somehow guaranteed that the current hart has exclusive access
> >> to *rscratch even at the beginning of sbi_hsm_hart_start(), then yes, we
> >> can drop aux_state, etc., and use a simpler fix.
> >>
> >> smp_wmb() will then be paired with rmb() in atomic_read(&hdata->state)
> >> from sbi_hsm_hart_wait() and it would make sure the writes are seen in
> >> correct order.
> >>
> >> So, should we worry about misbehaving OSes which could allow 2 or more
> >> harts to enter sbi_hsm_hart_start() for the same target hart in parallel?
> >>
> >> What do you think?
> >
> > I see your point but it is better to tackle the situation of 2 or more
> > harts entering sbi_hsm_hart_start() rather than aux_state which
> > less readable.
> >
> > How about this ?
> >
> > 1) Re-base on-top of the below patch
> > 2) Update this patch to:
> > a) Move "rscratch->next_*" updates before
> > "hdata = sbi_scratch_offset_ptr(rscratch, hart_data_offset);"
> > in sbi_hsm_hart_start()
> > b) Add "start_ticket" which needs to be acquired (0->1) before
> > "rscratch->next_*" updates in sbi_hsm_hart_start() and
> > if a HART fails to acquire it then sbi_hsm_hart_start() fails
> > c) Read the "scratch->next_" data and release "start_ticket" (1->0)
> > in sbi_hsm_hart_start_finish() before doing sbi_hart_switch_mode().
> >
>
> If 'start_ticket' is a global, rather than a per-hart variable, no hart
> would be allowed to run sbi_hsm_hart_start() till the secondary hart
> finishes init_warm_startup() and releases the ticket. This could prevent
> the boot hart to call sbi_hsm_hart_start() for other harts, e.g. if the
> boot hart executes sbi_hsm_hart_start() quickly, while the secondary
> hart is slow.
>
> I think, it would be sufficient to prevent entering sbi_hsm_hart_start()
> only for the *same* target hart rather than for *all* target harts.
>
> If it is OK to add fields to 'struct sbi_hsm_data', we could add
> 'start_ticket' there, instead of 'aux_data', and use it as you suggested.
>
> Would that be OK?
Sorry, I was not clear previously but I assumed "start_ticket" to be
per-HART. It should be added in 'struct sbi_hsm_data'
Regards,
Anup
>
> Regards,
> Evgenii
>
> > Regards,
> > Anup
> >
> > diff --git a/include/sbi/sbi_hsm.h b/include/sbi/sbi_hsm.h
> > index c0b4830..4b5601b 100644
> > --- a/include/sbi/sbi_hsm.h
> > +++ b/include/sbi/sbi_hsm.h
> > @@ -66,7 +66,8 @@ int sbi_hsm_hart_start(struct sbi_scratch *scratch,
> > u32 hartid, ulong saddr, ulong smode, ulong arg1);
> > int sbi_hsm_hart_stop(struct sbi_scratch *scratch, bool exitnow);
> > void sbi_hsm_hart_resume_start(struct sbi_scratch *scratch);
> > -void sbi_hsm_hart_resume_finish(struct sbi_scratch *scratch);
> > +void __noreturn sbi_hsm_hart_resume_finish(struct sbi_scratch *scratch,
> > + u32 hartid);
> > int sbi_hsm_hart_suspend(struct sbi_scratch *scratch, u32 suspend_type,
> > ulong raddr, ulong rmode, ulong arg1);
> > bool sbi_hsm_hart_change_state(struct sbi_scratch *scratch, long oldstate,
> > @@ -76,6 +77,7 @@ int sbi_hsm_hart_get_state(const struct sbi_domain
> > *dom, u32 hartid);
> > int sbi_hsm_hart_interruptible_mask(const struct sbi_domain *dom,
> > ulong hbase, ulong *out_hmask);
> > void __sbi_hsm_suspend_non_ret_save(struct sbi_scratch *scratch);
> > -void sbi_hsm_prepare_next_jump(struct sbi_scratch *scratch, u32 hartid);
> > +void __noreturn sbi_hsm_hart_start_finish(struct sbi_scratch *scratch,
> > + u32 hartid);
> >
> > #endif
> > diff --git a/lib/sbi/sbi_hsm.c b/lib/sbi/sbi_hsm.c
> > index 3eeeb58..100b8c0 100644
> > --- a/lib/sbi/sbi_hsm.c
> > +++ b/lib/sbi/sbi_hsm.c
> > @@ -110,7 +110,8 @@ int sbi_hsm_hart_interruptible_mask(const struct
> > sbi_domain *dom,
> > return 0;
> > }
> >
> > -void sbi_hsm_prepare_next_jump(struct sbi_scratch *scratch, u32 hartid)
> > +void __noreturn sbi_hsm_hart_start_finish(struct sbi_scratch *scratch,
> > + u32 hartid)
> > {
> > struct sbi_hsm_data *hdata = sbi_scratch_offset_ptr(scratch,
> > hart_data_offset);
> > @@ -118,6 +119,9 @@ void sbi_hsm_prepare_next_jump(struct sbi_scratch
> > *scratch, u32 hartid)
> > if (!__sbi_hsm_hart_change_state(hdata, SBI_HSM_STATE_START_PENDING,
> > SBI_HSM_STATE_STARTED))
> > sbi_hart_hang();
> > +
> > + sbi_hart_switch_mode(hartid, scratch->next_arg1, scratch->next_addr,
> > + scratch->next_mode, false);
> > }
> >
> > static void sbi_hsm_hart_wait(struct sbi_scratch *scratch, u32 hartid)
> > @@ -381,7 +385,8 @@ void sbi_hsm_hart_resume_start(struct sbi_scratch *scratch)
> > hsm_device_hart_resume();
> > }
> >
> > -void sbi_hsm_hart_resume_finish(struct sbi_scratch *scratch)
> > +void __noreturn sbi_hsm_hart_resume_finish(struct sbi_scratch *scratch,
> > + u32 hartid)
> > {
> > struct sbi_hsm_data *hdata = sbi_scratch_offset_ptr(scratch,
> > hart_data_offset);
> > @@ -396,6 +401,10 @@ void sbi_hsm_hart_resume_finish(struct
> > sbi_scratch *scratch)
> > * the warm-boot sequence.
> > */
> > __sbi_hsm_suspend_non_ret_restore(scratch);
> > +
> > + sbi_hart_switch_mode(hartid, scratch->next_arg1,
> > + scratch->next_addr,
> > + scratch->next_mode, false);
> > }
> >
> > int sbi_hsm_hart_suspend(struct sbi_scratch *scratch, u32 suspend_type,
> > diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
> > index bc60a42..dcca2c8 100644
> > --- a/lib/sbi/sbi_init.c
> > +++ b/lib/sbi/sbi_init.c
> > @@ -355,12 +355,11 @@ static void __noreturn init_coldboot(struct
> > sbi_scratch *scratch, u32 hartid)
> > init_count = sbi_scratch_offset_ptr(scratch, init_count_offset);
> > (*init_count)++;
> >
> > - sbi_hsm_prepare_next_jump(scratch, hartid);
> > - sbi_hart_switch_mode(hartid, scratch->next_arg1, scratch->next_addr,
> > - scratch->next_mode, false);
> > + sbi_hsm_hart_start_finish(scratch, hartid);
> > }
> >
> > -static void init_warm_startup(struct sbi_scratch *scratch, u32 hartid)
> > +static void __noreturn init_warm_startup(struct sbi_scratch *scratch,
> > + u32 hartid)
> > {
> > int rc;
> > unsigned long *init_count;
> > @@ -412,10 +411,11 @@ static void init_warm_startup(struct sbi_scratch
> > *scratch, u32 hartid)
> > init_count = sbi_scratch_offset_ptr(scratch, init_count_offset);
> > (*init_count)++;
> >
> > - sbi_hsm_prepare_next_jump(scratch, hartid);
> > + sbi_hsm_hart_start_finish(scratch, hartid);
> > }
> >
> > -static void init_warm_resume(struct sbi_scratch *scratch)
> > +static void __noreturn init_warm_resume(struct sbi_scratch *scratch,
> > + u32 hartid)
> > {
> > int rc;
> >
> > @@ -429,7 +429,7 @@ static void init_warm_resume(struct sbi_scratch *scratch)
> > if (rc)
> > sbi_hart_hang();
> >
> > - sbi_hsm_hart_resume_finish(scratch);
> > + sbi_hsm_hart_resume_finish(scratch, hartid);
> > }
> >
> > static void __noreturn init_warmboot(struct sbi_scratch *scratch, u32 hartid)
> > @@ -443,13 +443,9 @@ static void __noreturn init_warmboot(struct
> > sbi_scratch *scratch, u32 hartid)
> > sbi_hart_hang();
> >
> > if (hstate == SBI_HSM_STATE_SUSPENDED)
> > - init_warm_resume(scratch);
> > + init_warm_resume(scratch, hartid);
> > else
> > init_warm_startup(scratch, hartid);
> > -
> > - sbi_hart_switch_mode(hartid, scratch->next_arg1,
> > - scratch->next_addr,
> > - scratch->next_mode, false);
> > }
> >
> > static atomic_t coldboot_lottery = ATOMIC_INITIALIZER(0);
> >
>
>
>
> --
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