Re: [EXTERNAL] Re: [PATCH 5/9] x86/hyperv: Use vmmcall to implement Hyper-V hypercall in sev-snp enlightened guest

Tianyu Lan
2023-06-08T15:15:47Z

On 6/8/2023 9:21 PM, Peter Zijlstra wrote:
> On Thu, Jun 01, 2023 at 11:16:18AM -0400, Tianyu Lan wrote:
>> From: Tianyu Lan <tiala@microsoft.com>
>>
>> In sev-snp enlightened guest, Hyper-V hypercall needs
>> to use vmmcall to trigger vmexit and notify hypervisor
>> to handle hypercall request.
>>
>> There is no x86 SEV SNP feature flag support so far and
>> hardware provides MSR_AMD64_SEV register to check SEV-SNP
>> capability with MSR_AMD64_SEV_ENABLED bit. ALTERNATIVE can't
>> work without SEV-SNP x86 feature flag. May add later when
>> the associated flag is introduced.
>>
>> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
>> ---
>> arch/x86/include/asm/mshyperv.h | 44 ++++++++++++++++++++++++---------
>> 1 file changed, 33 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
>> index 31c476f4e656..d859d7c5f5e8 100644
>> --- a/arch/x86/include/asm/mshyperv.h
>> +++ b/arch/x86/include/asm/mshyperv.h
>> @@ -61,16 +61,25 @@ static inline u64 hv_do_hypercall(u64 control, void *input, void *output)
>> u64 hv_status;
>>
>> #ifdef CONFIG_X86_64
>> - if (!hv_hypercall_pg)
>> - return U64_MAX;
>> + if (hv_isolation_type_en_snp()) {
>> + __asm__ __volatile__("mov %4, %%r8\n"
>> + "vmmcall"
>> + : "=a" (hv_status), ASM_CALL_CONSTRAINT,
>> + "+c" (control), "+d" (input_address)
>> + : "r" (output_address)
>> + : "cc", "memory", "r8", "r9", "r10", "r11");
>> + } else {
>> + if (!hv_hypercall_pg)
>> + return U64_MAX;
>>
>> - __asm__ __volatile__("mov %4, %%r8\n"
>> - CALL_NOSPEC
>> - : "=a" (hv_status), ASM_CALL_CONSTRAINT,
>> - "+c" (control), "+d" (input_address)
>> - : "r" (output_address),
>> - THUNK_TARGET(hv_hypercall_pg)
>> - : "cc", "memory", "r8", "r9", "r10", "r11");
>> + __asm__ __volatile__("mov %4, %%r8\n"
>> + CALL_NOSPEC
>> + : "=a" (hv_status), ASM_CALL_CONSTRAINT,
>> + "+c" (control), "+d" (input_address)
>> + : "r" (output_address),
>> + THUNK_TARGET(hv_hypercall_pg)
>> + : "cc", "memory", "r8", "r9", "r10", "r11");
>> + }
>> #else
>
> Remains unanswered:
>
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20230516102912.GG2587705%2540hirez.programming.kicks-ass.net&data=05%7C01%7CTianyu.Lan%40microsoft.com%7C60a576eb67634ffa27b108db68234d5a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C638218273105649705%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=MFj67DON0K%2BUoUJbeaIA5oVTxyrzO3fb5DbxYgDWwX0%3D&reserved=0
>
> Would this not generate better code with an alternative?

Hi Peter:
Thanks to review. I put the explaination in the change log.

"There is no x86 SEV SNP feature(X86_FEATURE_SEV_SNP) flag
support so far and hardware provides MSR_AMD64_SEV register
to check SEV-SNP capability with MSR_AMD64_SEV_ENABLED bit
ALTERNATIVE can't work without SEV-SNP x86 feature flag."
There is no cpuid leaf bit to check AMD SEV-SNP feature.

After some Hyper-V doesn't provides SEV and SEV-ES guest before and so
may reuse X86_FEATURE_SEV and X86_FEATURE_SEV_ES flag as alternative
feature check for Hyper-V SEV-SNP guest. Will refresh patch.