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Date: Wed, 2 Dec 2020 16:34:12 -0800 From: Sean Christopherson <seanjc@...gle.com> To: Ashish Kalra <Ashish.Kalra@....com> Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, hpa@...or.com, joro@...tes.org, bp@...e.de, thomas.lendacky@....com, x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, srutherford@...gle.com, brijesh.singh@....com, dovmurik@...ux.vnet.ibm.com, tobin@....com, jejb@...ux.ibm.com, frankeh@...ibm.com, dgilbert@...hat.com Subject: Re: [PATCH v2 1/9] KVM: x86: Add AMD SEV specific Hypercall3 On Tue, Dec 01, 2020, Ashish Kalra wrote: > From: Brijesh Singh <brijesh.singh@....com> > > 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. > Cc: Thomas Gleixner <tglx@...utronix.de> > Cc: Ingo Molnar <mingo@...hat.com> > Cc: "H. Peter Anvin" <hpa@...or.com> > Cc: Paolo Bonzini <pbonzini@...hat.com> > Cc: "Radim Krčmář" <rkrcmar@...hat.com> > Cc: Joerg Roedel <joro@...tes.org> > Cc: Borislav Petkov <bp@...e.de> > Cc: Tom Lendacky <thomas.lendacky@....com> > Cc: x86@...nel.org > Cc: kvm@...r.kernel.org > Cc: linux-kernel@...r.kernel.org > Reviewed-by: Steve Rutherford <srutherford@...gle.com> > Reviewed-by: Venu Busireddy <venu.busireddy@...cle.com> > Signed-off-by: Brijesh Singh <brijesh.singh@....com> > Signed-off-by: Ashish Kalra <ashish.kalra@....com> > --- > 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 >
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