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SubjectRe: [PATCH v3] KVM: VMX: do not disable interception for MSR_IA32_SPEC_CTRL on eIBRS
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> On May 25, 2022, at 1:04 PM, Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, May 20, 2022, Jon Kohler wrote:
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 610355b9ccce..1c725d17d984 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -2057,20 +2057,32 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>> return 1;
>>
>> vmx->spec_ctrl = data;
>> - if (!data)
>> +
>> + /*
>> + * Disable interception on the first non-zero write, unless the
>> + * guest is hosted on an eIBRS system and setting only
>
> The "unless guest is hosted on an eIBRS system" blurb is wrong and doesn't match

Ah right, thanks for catching that

> the code. Again, it's all about whether eIBRS is advertised to the guest. With
> some other minor tweaking to wrangle the comment to 80 chars...

RE 80 chars - quick question (and forgive the silly question here), but how are you
counting that? I’ve got my editor cutting at 79 cols, where tab size is accounted
for as 4 cols, so the longest line on my side for this patch is 72-73 or so.

These also pass the checkpatch.pl script as well, so I just want to make sure
going forward I’m formatting them appropriately.

Let me know and I’ll incorporate all of this into a v4 after I hear back :)

>
> /*
> * Disable interception on the first non-zero write, except if
> * eIBRS is advertised to the guest and the guest is enabling
> * _only_ IBRS. On eIBRS systems, kernels typically set IBRS
> * once at boot and never touch it post-boot. All other bits,
> * and IBRS on non-eIBRS systems, are often set on a per-task
> * basis, i.e. change frequently, so the benefit of avoiding
> * VM-exits during guest context switches outweighs the cost of
> * RDMSR on every VM-Exit to save the guest's value.
> */

Thanks for the suggestion, this text works for me

>
>> + * SPEC_CTRL_IBRS, which is typically set once at boot and never
>
> Uber nit, when it makes sense, avoid regurgitating the code verbatim, e.g. refer
> to "setting SPEC_CTRL_IBRS" as "enabling IBRS". That little bit of abstraction
> can sometimes help unfamiliar readers understand the effect of the code, whereas
> copy+pasting bits of the code doesn't provide any additional context.

Ok thanks for this as well, I appreciate the feedback!

>
>> + * touched again. All other bits are often set on a per-task
>> + * basis, i.e. may change frequently, so the benefit of avoiding
>> + * VM-exits during guest context switches outweighs the cost of
>> + * RDMSR on every VM-Exit to save the guest's value.
>> + */
>> + if (!data ||
>> + (data == SPEC_CTRL_IBRS &&
>> + (vcpu->arch.arch_capabilities & ARCH_CAP_IBRS_ALL)))
>
> Align the two halves of the logical-OR, i.e.
>
> if (!data ||
> (data == SPEC_CTRL_IBRS &&
> (vcpu->arch.arch_capabilities & ARCH_CAP_IBRS_ALL)))
> break;

Ack on this

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