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

[Tianyu Lan]

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@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > ---
> >   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:
>
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>
> 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.