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

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Date:   Sun, 6 Dec 2020 11:26:12 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Ashish Kalra <Ashish.Kalra@....com>
Cc:     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 03/12/20 01:34, Sean Christopherson wrote:
> 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.

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

Paolo

> 
>> 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
>>
>