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

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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
>