[PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Tom Lendacky]

From: Tom Lendacky <thomas.lendacky@amd.com>

The INVD instruction intercept performs emulation. Emulation can't be done
on an SEV guest because the guest memory is encrypted.

Provide a dedicated intercept routine for the INVD intercept. Within this
intercept routine just skip the instruction for an SEV guest, since it is
emulated as a NOP anyway.

Fixes: 1654efcbc431 ("KVM: SVM: Add KVM_SEV_INIT command")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 arch/x86/kvm/svm/svm.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index c91acabf18d0..332ec4425d89 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2183,6 +2183,17 @@ static int iret_interception(struct vcpu_svm *svm)
 	return 1;
 }
 
+static int invd_interception(struct vcpu_svm *svm)
+{
+	/*
+	 * Can't do emulation on an SEV guest and INVD is emulated
+	 * as a NOP, so just skip the instruction.
+	 */
+	return (sev_guest(svm->vcpu.kvm))
+		? kvm_skip_emulated_instruction(&svm->vcpu)
+		: kvm_emulate_instruction(&svm->vcpu, 0);
+}
+
 static int invlpg_interception(struct vcpu_svm *svm)
 {
 	if (!static_cpu_has(X86_FEATURE_DECODEASSISTS))
@@ -2774,7 +2785,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
 	[SVM_EXIT_RDPMC]			= rdpmc_interception,
 	[SVM_EXIT_CPUID]			= cpuid_interception,
 	[SVM_EXIT_IRET]                         = iret_interception,
-	[SVM_EXIT_INVD]                         = emulate_on_interception,
+	[SVM_EXIT_INVD]                         = invd_interception,
 	[SVM_EXIT_PAUSE]			= pause_interception,
 	[SVM_EXIT_HLT]				= halt_interception,
 	[SVM_EXIT_INVLPG]			= invlpg_interception,
-- 
2.28.0

Re: [PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Sean Christopherson]

On Wed, Sep 23, 2020 at 03:27:39PM -0500, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> The INVD instruction intercept performs emulation. Emulation can't be done
> on an SEV guest because the guest memory is encrypted.
> 
> Provide a dedicated intercept routine for the INVD intercept. Within this
> intercept routine just skip the instruction for an SEV guest, since it is
> emulated as a NOP anyway.
> 
> Fixes: 1654efcbc431 ("KVM: SVM: Add KVM_SEV_INIT command")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/kvm/svm/svm.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index c91acabf18d0..332ec4425d89 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2183,6 +2183,17 @@ static int iret_interception(struct vcpu_svm *svm)
>  	return 1;
>  }
>  
> +static int invd_interception(struct vcpu_svm *svm)
> +{
> +	/*
> +	 * Can't do emulation on an SEV guest and INVD is emulated
> +	 * as a NOP, so just skip the instruction.
> +	 */
> +	return (sev_guest(svm->vcpu.kvm))
> +		? kvm_skip_emulated_instruction(&svm->vcpu)
> +		: kvm_emulate_instruction(&svm->vcpu, 0);

Is there any reason not to do kvm_skip_emulated_instruction() for both SEV
and legacy?  VMX has the same odd kvm_emulate_instruction() call, but AFAICT
that's completely unecessary, i.e. VMX can also convert to a straight skip.

> +}
> +
>  static int invlpg_interception(struct vcpu_svm *svm)
>  {
>  	if (!static_cpu_has(X86_FEATURE_DECODEASSISTS))
> @@ -2774,7 +2785,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
>  	[SVM_EXIT_RDPMC]			= rdpmc_interception,
>  	[SVM_EXIT_CPUID]			= cpuid_interception,
>  	[SVM_EXIT_IRET]                         = iret_interception,
> -	[SVM_EXIT_INVD]                         = emulate_on_interception,
> +	[SVM_EXIT_INVD]                         = invd_interception,
>  	[SVM_EXIT_PAUSE]			= pause_interception,
>  	[SVM_EXIT_HLT]				= halt_interception,
>  	[SVM_EXIT_INVLPG]			= invlpg_interception,
> -- 
> 2.28.0
> 

Re: [PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Tom Lendacky]

On 9/23/20 3:32 PM, Sean Christopherson wrote:
> On Wed, Sep 23, 2020 at 03:27:39PM -0500, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> The INVD instruction intercept performs emulation. Emulation can't be done
>> on an SEV guest because the guest memory is encrypted.
>>
>> Provide a dedicated intercept routine for the INVD intercept. Within this
>> intercept routine just skip the instruction for an SEV guest, since it is
>> emulated as a NOP anyway.
>>
>> Fixes: 1654efcbc431 ("KVM: SVM: Add KVM_SEV_INIT command")
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  arch/x86/kvm/svm/svm.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index c91acabf18d0..332ec4425d89 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -2183,6 +2183,17 @@ static int iret_interception(struct vcpu_svm *svm)
>>  	return 1;
>>  }
>>  
>> +static int invd_interception(struct vcpu_svm *svm)
>> +{
>> +	/*
>> +	 * Can't do emulation on an SEV guest and INVD is emulated
>> +	 * as a NOP, so just skip the instruction.
>> +	 */
>> +	return (sev_guest(svm->vcpu.kvm))
>> +		? kvm_skip_emulated_instruction(&svm->vcpu)
>> +		: kvm_emulate_instruction(&svm->vcpu, 0);
> 
> Is there any reason not to do kvm_skip_emulated_instruction() for both SEV
> and legacy?  VMX has the same odd kvm_emulate_instruction() call, but AFAICT
> that's completely unecessary, i.e. VMX can also convert to a straight skip.

You could, I just figured I'd leave the legacy behavior just in case. Not
that I can think of a reason that behavior would ever change.

Thanks,
Tom

> 
>> +}
>> +
>>  static int invlpg_interception(struct vcpu_svm *svm)
>>  {
>>  	if (!static_cpu_has(X86_FEATURE_DECODEASSISTS))
>> @@ -2774,7 +2785,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
>>  	[SVM_EXIT_RDPMC]			= rdpmc_interception,
>>  	[SVM_EXIT_CPUID]			= cpuid_interception,
>>  	[SVM_EXIT_IRET]                         = iret_interception,
>> -	[SVM_EXIT_INVD]                         = emulate_on_interception,
>> +	[SVM_EXIT_INVD]                         = invd_interception,
>>  	[SVM_EXIT_PAUSE]			= pause_interception,
>>  	[SVM_EXIT_HLT]				= halt_interception,
>>  	[SVM_EXIT_INVLPG]			= invlpg_interception,
>> -- 
>> 2.28.0
>>

Re: [PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Paolo Bonzini]

On 23/09/20 22:40, Tom Lendacky wrote:
>>> +static int invd_interception(struct vcpu_svm *svm)
>>> +{
>>> +	/*
>>> +	 * Can't do emulation on an SEV guest and INVD is emulated
>>> +	 * as a NOP, so just skip the instruction.
>>> +	 */
>>> +	return (sev_guest(svm->vcpu.kvm))
>>> +		? kvm_skip_emulated_instruction(&svm->vcpu)
>>> +		: kvm_emulate_instruction(&svm->vcpu, 0);
>>
>> Is there any reason not to do kvm_skip_emulated_instruction() for both SEV
>> and legacy?  VMX has the same odd kvm_emulate_instruction() call, but AFAICT
>> that's completely unecessary, i.e. VMX can also convert to a straight skip.
> 
> You could, I just figured I'd leave the legacy behavior just in case. Not
> that I can think of a reason that behavior would ever change.

Yeah, let's do skip for both SVM and VMX.

Paolo

Re: [PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Tom Lendacky]

On 9/24/20 1:51 AM, Paolo Bonzini wrote:
> On 23/09/20 22:40, Tom Lendacky wrote:
>>>> +static int invd_interception(struct vcpu_svm *svm)
>>>> +{
>>>> +	/*
>>>> +	 * Can't do emulation on an SEV guest and INVD is emulated
>>>> +	 * as a NOP, so just skip the instruction.
>>>> +	 */
>>>> +	return (sev_guest(svm->vcpu.kvm))
>>>> +		? kvm_skip_emulated_instruction(&svm->vcpu)
>>>> +		: kvm_emulate_instruction(&svm->vcpu, 0);
>>>
>>> Is there any reason not to do kvm_skip_emulated_instruction() for both SEV
>>> and legacy?  VMX has the same odd kvm_emulate_instruction() call, but AFAICT
>>> that's completely unecessary, i.e. VMX can also convert to a straight skip.
>>
>> You could, I just figured I'd leave the legacy behavior just in case. Not
>> that I can think of a reason that behavior would ever change.
> 
> Yeah, let's do skip for both SVM and VMX.

Ok, I'll submit a two patch series to change SVM and VMX. I'll do two 
patches because of the fixes tag to get the SVM fix back to stable. But, 
if you would prefer a single patch, let me know.

Thanks,
Tom

> 
> Paolo
> 

Re: [PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Vitaly Kuznetsov]

Tom Lendacky <thomas.lendacky@amd.com> writes:

> From: Tom Lendacky <thomas.lendacky@amd.com>
>
> The INVD instruction intercept performs emulation. Emulation can't be done
> on an SEV guest because the guest memory is encrypted.
>
> Provide a dedicated intercept routine for the INVD intercept. Within this
> intercept routine just skip the instruction for an SEV guest, since it is
> emulated as a NOP anyway.
>
> Fixes: 1654efcbc431 ("KVM: SVM: Add KVM_SEV_INIT command")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/kvm/svm/svm.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index c91acabf18d0..332ec4425d89 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2183,6 +2183,17 @@ static int iret_interception(struct vcpu_svm *svm)
>  	return 1;
>  }
>  
> +static int invd_interception(struct vcpu_svm *svm)
> +{
> +	/*
> +	 * Can't do emulation on an SEV guest and INVD is emulated
> +	 * as a NOP, so just skip the instruction.
> +	 */
> +	return (sev_guest(svm->vcpu.kvm))
> +		? kvm_skip_emulated_instruction(&svm->vcpu)
> +		: kvm_emulate_instruction(&svm->vcpu, 0);
> +}
> +
>  static int invlpg_interception(struct vcpu_svm *svm)
>  {
>  	if (!static_cpu_has(X86_FEATURE_DECODEASSISTS))
> @@ -2774,7 +2785,7 @@ static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
>  	[SVM_EXIT_RDPMC]			= rdpmc_interception,
>  	[SVM_EXIT_CPUID]			= cpuid_interception,
>  	[SVM_EXIT_IRET]                         = iret_interception,
> -	[SVM_EXIT_INVD]                         = emulate_on_interception,
> +	[SVM_EXIT_INVD]                         = invd_interception,
>  	[SVM_EXIT_PAUSE]			= pause_interception,
>  	[SVM_EXIT_HLT]				= halt_interception,
>  	[SVM_EXIT_INVLPG]			= invlpg_interception,

Out of pure curiosity,

does it sill make sense to intercept INVD when we just skip it? Would it
rather make sense to disable INVD intercept for SEV guests completely?

-- 
Vitaly

Re: [PATCH] KVM: SVM: Add a dedicated INVD intercept routine

[Paolo Bonzini]

On 24/09/20 15:58, Vitaly Kuznetsov wrote:
> does it sill make sense to intercept INVD when we just skip it? Would it
> rather make sense to disable INVD intercept for SEV guests completely?

If we don't intercept the processor would really invalidate the cache,
that is certainly not what we want.

Paolo