[RFC PATCH] arm64: KVM: honor cacheability attributes on S2 page fault

[Anup Patel]

On Fri, Oct 11, 2013 at 6:08 PM, Catalin Marinas
<catalin.marinas at arm.com> wrote:
> On Thu, Oct 10, 2013 at 05:09:03PM +0100, Anup Patel wrote:
>> On Thu, Oct 10, 2013 at 4:54 PM, Catalin Marinas
>> <catalin.marinas at arm.com> wrote:
>> > On Thu, Oct 10, 2013 at 09:39:55AM +0100, Marc Zyngier wrote:
>> >> On 10/10/13 05:51, Anup Patel wrote:
>> >> > Are you planning to go ahead with this approach ?
>> >>
>> >> [adding Catalin, as we heavily discussed this recently]
>> >>
>> >> Not as such, as it doesn't solve the full issue. It merely papers over
>> >> the whole "my cache is off" problem. More specifically, any kind of
>> >> speculative access from another CPU while caches are off in the guest
>> >> completely nukes the benefit of this patch.
>> >>
>> >> Also, turning the the caches off is another source of problems, as
>> >> speculation also screws up set/way invalidation.
>> >
>> > Indeed. The set/way operations trapping and broadcasting (or deferring)
>> > to other CPUs in software just happens to work but there is no
>> > guarantee, sooner or later we'll hit a problem. I'm even tempted to
>> > remove flush_dcache_all() calls on the booting path for the arm64
>> > kernel, we already require that whatever runs before Linux should
>> > clean&invalidate the caches.
>> >
>> > Basically, with KVM a VCPU even if running with caches/MMU disabled can
>> > still get speculative allocation into the cache. The reason for this is
>> > the other cacheable memory aliases created by the host kernel and
>> > qemu/kvmtool. I can't tell whether Xen has this issue but it may be
>> > easier in Xen to avoid memory aliases.
>> >
>> >> > We really need this patch for X-Gene L3 cache.
>> >>
>> >> So far, I can see two possibilities:
>> >> - either we mandate caches to be always on (DC bit, and you're not
>> >> allowed to turn the caches off).
>> >
>> > That's my preferred approach. For hotplug, idle, the guest would use an
>> > HVC call (PSCI) and the host takes care of re-enabling the DC bit. But
>> > we may not catch all cases (kexec probably).
>> >
>> >> - Or we mandate that caches are invalidated (by VA) for each write that
>> >> is performed with caches off.
>> >
>> > For some things like run-time code patching, on ARMv8 we need to do at
>> > least I-cache maintenance since the CPU can allocate into the I-cache
>> > (even if there are no aliases).
>>
>> It seems all approaches considered so far have a corner case in
>> one-way or another.
>
> Yes, we try to settle on the one with least corner cases.

Ok.

>
>> Coming back to where we started, the actual problem was that when
>> Guest starts booting it sees wrong contents because it is runs with
>> MMU disable and correct contents are still in external L3 cache of X-Gene.
>
> That's one of the problems and I think the easiest to solve. Note that
> contents could still be in the L1/L2 (inner) cache since whole cache
> flushing by set/way isn't guaranteed in an MP context.
>
>> How about reconsidering the approach of flushing Guest RAM (entire or
>> portion of it) to PoC by VA once before the first run of a VCPU ?
>
> Flushing the entire guest RAM is not possible by set/way
> (architecturally) and not efficient by VA (though some benchmark would
> be good). Marc's patch defers this flushing when a page is faulted in
> (at stage 2) and I think it covers the initial boot.
>
>> OR
>> We can also have KVM API using which user space can flush portions
>> of Guest RAM before running the VCPU. (I think this was a suggestion
>> from Marc Z initially)
>
> This may not be enough. It indeed flushes the kernel image that gets
> loaded but the kernel would write other pages (bss, page tables etc.)
> with MMU disabled and those addresses may contain dirty cache lines that
> have not been covered by the initial kvmtool flush. So you basically
> need all guest non-cacheable accesses to be flushed.
>
> The other problems are the cacheable aliases that I mentioned, so even
> though the guest does non-cacheable accesses with the MMU off, the
> hardware can still allocate into the cache via the other mappings. In
> this case the guest needs to invalidate the areas of memory that it
> wrote with caches off (or just use the DC bit to force memory accesses
> with MMU off to be cacheable).

Having looked at all the approaches, I would vote for the approach taken
by this patch.

>
> --
> Catalin

--Anup