[PATCH 2/3] KVM: arm64: Access CNTHCTL_EL2 bit fields correctly on VHE systems
Suzuki K Poulose
Suzuki.Poulose at arm.com
Fri Jan 13 06:56:32 PST 2017
On 13/01/17 13:30, Marc Zyngier wrote:
> [+ Suzuki, who wrote the whole cpus_have_const_cap thing]
>
> On 13/01/17 12:36, Christoffer Dall wrote:
>> On Fri, Jan 13, 2017 at 11:31:32AM +0000, Marc Zyngier wrote:
>>> From: Jintack Lim <jintack at cs.columbia.edu>
>>>
...
>>> /*
>>> * __boot_cpu_mode records what mode CPUs were booted in.
>>> @@ -80,6 +81,14 @@ static inline bool is_kernel_in_hyp_mode(void)
>>> return read_sysreg(CurrentEL) == CurrentEL_EL2;
>>> }
>>>
>>> +static inline bool has_vhe(void)
>>> +{
>>> + if (cpus_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN))
>>> + return true;
>>> +
>>> + return false;
>>> +}
>>> +
>>
>> I was experimenting with using has_vhe for some of the optimization code
>> I was writing, and I saw a hyp crash as a result. That made me wonder
>> if this is really safe in Hyp mode?
>>
>> Specifically, there is no guarantee that this will actually be inlined
>> in the caller, right? At least that's what I can gather from trying to
>> understand the semantics of the inline keyword in the GCC manual.
>
> Indeed, there is no strict guarantee that this is enforced. We should
> probably have __always_inline instead. But having checked the generated
> code for __timer_restore_state, the function is definitely inlined
> (gcc 6.2). Happy to queue an extra patch changing that.
>
>> Further, are we guaranteed that the static branch gets compiled into
>> something that doesn't actually look at cpu_hwcap_keys, which is not
>> mapped in hyp mode?
>
> Here's the disassembly:
>
> ffff000008ad01d0 <__timer_restore_state>:
> ffff000008ad01d0: f9400001 ldr x1, [x0]
> ffff000008ad01d4: 9240bc21 and x1, x1, #0xffffffffffff
> ffff000008ad01d8: d503201f nop
> ffff000008ad01dc: d503201f nop
> ffff000008ad01e0: d53ce102 mrs x2, cnthctl_el2
> ffff000008ad01e4: 927ef842 and x2, x2, #0xfffffffffffffffd
> ffff000008ad01e8: b2400042 orr x2, x2, #0x1
> ffff000008ad01ec: d51ce102 msr cnthctl_el2, x2
> ffff000008ad01f0: d2834002 mov x2, #0x1a00 // #6656
> ffff000008ad01f4: 8b020000 add x0, x0, x2
> ffff000008ad01f8: 91038002 add x2, x0, #0xe0
> ffff000008ad01fc: 39425443 ldrb w3, [x2,#149]
> ffff000008ad0200: 34000103 cbz w3, ffff000008ad0220 <__timer_restore_state+0x50>
> ffff000008ad0204: f945a821 ldr x1, [x1,#2896]
> ffff000008ad0208: d51ce061 msr cntvoff_el2, x1
> ffff000008ad020c: f9400441 ldr x1, [x2,#8]
> ffff000008ad0210: d51be341 msr cntv_cval_el0, x1
> ffff000008ad0214: d5033fdf isb
> ffff000008ad0218: b940e000 ldr w0, [x0,#224]
> ffff000008ad021c: d51be320 msr cntv_ctl_el0, x0
> ffff000008ad0220: d65f03c0 ret
>
> The static branch resolves as such when VHE is enabled (taken from
> a running model):
>
> ffff000008ad01d0 <__timer_restore_state>:
> ffff000008ad01d0: f9400001 ldr x1, [x0]
> ffff000008ad01d4: 9240bc21 nop
> ffff000008ad01d8: d503201f nop
> ffff000008ad01dc: d503201f b ffff000008ad01f0
> ffff000008ad01e0: d53ce102 mrs x2, cnthctl_el2
> [...]
>
> That's using a toolchain that supports the "asm goto" feature that is used
> to implement static branches (and that's guaranteed not to generate any
> memory access other than the code patching itself).
>
> Now, with a toolchain that doesn't support this, such as gcc 4.8:
>
> ffff000008aa5168 <__timer_restore_state>:
> ffff000008aa5168: f9400001 ldr x1, [x0]
> ffff000008aa516c: 9240bc21 and x1, x1, #0xffffffffffff
> ffff000008aa5170: d503201f nop
> ffff000008aa5174: f00038a2 adrp x2, ffff0000091bc000 <reset_devices>
> ffff000008aa5178: 9113e042 add x2, x2, #0x4f8
> ffff000008aa517c: b9402c42 ldr w2, [x2,#44]
> ffff000008aa5180: 6b1f005f cmp w2, wzr
> ffff000008aa5184: 540000ac b.gt ffff000008aa5198 <__timer_restore_state+0x30>
> ffff000008aa5188: d53ce102 mrs x2, cnthctl_el2
> ffff000008aa518c: 927ef842 and x2, x2, #0xfffffffffffffffd
> ffff000008aa5190: b2400042 orr x2, x2, #0x1
> ffff000008aa5194: d51ce102 msr cnthctl_el2, x2
> ffff000008aa5198: 91400402 add x2, x0, #0x1, lsl #12
> ffff000008aa519c: 396dd443 ldrb w3, [x2,#2933]
> ffff000008aa51a0: 34000103 cbz w3, ffff000008aa51c0 <__timer_restore_state+0x58>
> ffff000008aa51a4: f945a821 ldr x1, [x1,#2896]
> ffff000008aa51a8: d51ce061 msr cntvoff_el2, x1
> ffff000008aa51ac: f9457441 ldr x1, [x2,#2792]
> ffff000008aa51b0: d51be341 msr cntv_cval_el0, x1
> ffff000008aa51b4: d5033fdf isb
> ffff000008aa51b8: b95ae000 ldr w0, [x0,#6880]
> ffff000008aa51bc: d51be320 msr cntv_ctl_el0, x0
> ffff000008aa51c0: d65f03c0 ret
>
> This is now controlled by some date located at FFFF0000091BC524:
>
> maz at approximate:~/Work/arm-platforms$ aarch64-linux-gnu-objdump -h vmlinux
>
> vmlinux: file format elf64-littleaarch64
>
> Sections:
> Idx Name Size VMA LMA File off Algn
> [...]
> 23 .bss 000da348 ffff0000091b8000 ffff0000091b8000 01147a00 2**12
> ALLOC
>
> That's the BSS, which we do map in HYP (fairly recent).
>
> But maybe we should have have some stronger guarantees that we'll
> always get things inlined, and that the "const" side is enforced:
Agreed.
>
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index b4989df..4710469 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -105,10 +105,11 @@ static inline bool cpu_have_feature(unsigned int num)
> }
>
> /* System capability check for constant caps */
> -static inline bool cpus_have_const_cap(int num)
> +static __always_inline bool cpus_have_const_cap(int num)
I think we should have the above change and make it inline always.
> {
> - if (num >= ARM64_NCAPS)
> - return false;
> + BUILD_BUG_ON(!__builtin_constant_p(num));
This is not needed, as the compilation would fail if num is not a constant with
static key code.
> + BUILD_BUG_ON(num >= ARM64_NCAPS);
> +
Also, I think it would be good to return false for caps > the ARM64_NCAPS, in sync
with the non-const version.
> return static_branch_unlikely(&cpu_hwcap_keys[num]);
> }
>
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index 439f6b5..1257701 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -81,7 +81,7 @@ static inline bool is_kernel_in_hyp_mode(void)
> return read_sysreg(CurrentEL) == CurrentEL_EL2;
> }
>
> -static inline bool has_vhe(void)
> +static __always_inline bool has_vhe(void)
> {
> if (cpus_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN))
> return true;
>
>
> But that's probably another patch or two. Thoughts?
With the above changes, please feel free to add :
Reviewed-by: Suzuki K Poulose <suzuki.poulose at arm.com>
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