Re: [PATCH v2 1/9] KVM: x86: Add AMD SEV specific Hypercall3

[Paolo Bonzini]

On 03/12/20 01:34, Sean Christopherson wrote:
> On Tue, Dec 01, 2020, Ashish Kalra wrote:
> > From: Brijesh Singh <brijesh.singh@xxxxxxx>
> >
> > KVM hypercall framework relies on alternative framework to patch the
> > VMCALL -> VMMCALL on AMD platform. If a hypercall is made before
> > apply_alternative() is called then it defaults to VMCALL. The approach
> > works fine on non SEV guest. A VMCALL would causes #UD, and hypervisor
> > will be able to decode the instruction and do the right things. But
> > when SEV is active, guest memory is encrypted with guest key and
> > hypervisor will not be able to decode the instruction bytes.
> >
> > Add SEV specific hypercall3, it unconditionally uses VMMCALL. The hypercall
> > will be used by the SEV guest to notify encrypted pages to the hypervisor.
>
> What if we invert KVM_HYPERCALL and X86_FEATURE_VMMCALL to default to VMMCALL
> and opt into VMCALL?  It's a synthetic feature flag either way, and I don't
> think there are any existing KVM hypercalls that happen before alternatives are
> patched, i.e. it'll be a nop for sane kernel builds.
>
> I'm also skeptical that a KVM specific hypercall is the right approach for the
> encryption behavior, but I'll take that up in the patches later in the series.

Do you think that it's the guest that should "donate" memory for the 
bitmap instead?

Paolo

> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
> > Cc: Joerg Roedel <joro@xxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: kvm@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Reviewed-by: Steve Rutherford <srutherford@xxxxxxxxxx>
> > Reviewed-by: Venu Busireddy <venu.busireddy@xxxxxxxxxx>
> > Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> > Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > ---
> >   arch/x86/include/asm/kvm_para.h | 12 ++++++++++++
> >   1 file changed, 12 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> > index 338119852512..bc1b11d057fc 100644
> > --- a/arch/x86/include/asm/kvm_para.h
> > +++ b/arch/x86/include/asm/kvm_para.h
> > @@ -85,6 +85,18 @@ static inline long kvm_hypercall4(unsigned int nr, unsigned long p1,
> >   	return ret;
> >   }
> >   
> > +static inline long kvm_sev_hypercall3(unsigned int nr, unsigned long p1,
> > +				      unsigned long p2, unsigned long p3)
> > +{
> > +	long ret;
> > +
> > +	asm volatile("vmmcall"
> > +		     : "=a"(ret)
> > +		     : "a"(nr), "b"(p1), "c"(p2), "d"(p3)
> > +		     : "memory");
> > +	return ret;
> > +}
> > +
> >   #ifdef CONFIG_KVM_GUEST
> >   bool kvm_para_available(void);
> >   unsigned int kvm_arch_para_features(void);
> > --
> > 2.17.1