Re: [RFC PATCH 28/35] KVM: X86: Update kvm_skip_emulated_instruction() for an SEV-ES guest

From: Tom Lendacky <thomas.lendacky@amd.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	x86@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Brijesh Singh <brijesh.singh@amd.com>
Subject: Re: [RFC PATCH 28/35] KVM: X86: Update kvm_skip_emulated_instruction() for an SEV-ES guest
Date: Tue, 15 Sep 2020 09:57:24 -0500	[thread overview]
Message-ID: <8cbafd62-528d-fc8e-9945-575c2b659247@amd.com> (raw)
In-Reply-To: <20200914215144.GE7192@sjchrist-ice>

On 9/14/20 4:51 PM, Sean Christopherson wrote:
> On Mon, Sep 14, 2020 at 03:15:42PM -0500, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> The register state for an SEV-ES guest is encrypted so the value of the
>> RIP cannot be updated. For an automatic exit, the RIP will be advanced
>> as necessary. For a non-automatic exit, it is up to the #VC handler in
>> the guest to advance the RIP.
>>
>> Add support to skip any RIP updates in kvm_skip_emulated_instruction()
>> for an SEV-ES guest.
> 
> Is there a reason this can't be handled in svm?  E.g. can KVM be reworked
> to effectively split the emulation logic so that it's a bug for KVM to end
> up trying to modify RIP?
> 
> Also, patch 06 modifies SVM's skip_emulated_instruction() to skip the RIP
> update, but keeps the "svm_set_interrupt_shadow(vcpu, 0)" logic.  Seems like
> either that change or this one is wrong.

I added this because of the get_rflags() call. But let me look into
changing get_rflags() in svm.c and see if this patch can go away.

Thanks,
Tom

> 
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  arch/x86/kvm/x86.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 23564d02d158..1dbdca607511 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -6874,13 +6874,17 @@ static int kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu)
>>  
>>  int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
>>  {
>> -	unsigned long rflags = kvm_x86_ops.get_rflags(vcpu);
>> +	unsigned long rflags;
>>  	int r;
>>  
>>  	r = kvm_x86_ops.skip_emulated_instruction(vcpu);
>>  	if (unlikely(!r))
>>  		return 0;
>>  
>> +	if (vcpu->arch.vmsa_encrypted)
>> +		return 1;
>> +
>> +	rflags = kvm_x86_ops.get_rflags(vcpu);
>>  	/*
>>  	 * rflags is the old, "raw" value of the flags.  The new value has
>>  	 * not been saved yet.
>> -- 
>> 2.28.0
>>