Re: [PATCH] KVM: x86: Allow exposure of VMware backdoor Pseudo-PMCs when !enable_pmu

[Like Xu]

Hi folks,

Please help confirm if VMware backdoor Pseudo-PMCs [*] are still available
when the KVM module parameter eneble_pmu=0.

	- VMWARE_BACKDOOR_PMC_HOST_TSC;
	- VMWARE_BACKDOOR_PMC_REAL_TIME;
	- VMWARE_BACKDOOR_PMC_APPARENT_TIME;

I'd like to say yes, but safely you have the official right to define it. Thanks.

[*] https://www.vmware.com/files/pdf/techpaper/Timekeeping-In-VirtualMachines.pdf

On 24/8/2023 4:35 am, Sean Christopherson wrote:
> On Tue, Aug 22, 2023, Like Xu wrote:
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > Fix kvm_pmu_rdpmc() logic to allow exposure of VMware backdoor Pseudo-PMCs
> > if pmu is globally disabled.
> >
> > In this usage scenario, once vmware_backdoor is enabled, the VMs can fully
> > utilize all of the vmware_backdoor-related resources, not just part of it,
> > i.e., vcpu should always be able to access the VMware backdoor Pseudo-PMCs
> > via RDPMC. Since the enable_pmu is more concerned with the visibility of
> > hardware pmu resources on the host, fix it to decouple the two knobs.
> >
> > The test case vmware_backdoors from KUT is used for validation.
>
> Is there a real world need for this?  Per commit 672ff6cff80c ("KVM: x86: Raise
> #GP when guest vCPU do not support PMU"), KVM's behavior is intentional.  If there
> is a real world need, then (a) that justification needs to be provided, (b) this
> needs a Fixes:, and (c) this probably needs to be tagged for stable.
>
> > Cc: Arbel Moshe <arbel.moshe@xxxxxxxxxx>
> > Cc: Liran Alon <liran.alon@xxxxxxxxxx>
> > Cc: Nikita Leshenko <nikita.leshchenko@xxxxxxxxxx>
>
> The expectation is that a Cc: in the changelog means the patch email is Cc'd to
> that person, i.e. one of the purposes of Cc: here is to record that people who
> might care about the patch have been made aware of its existence.  stable@ is
> pretty much the only exception to that rule, as "Cc: stable@xxxxxxxxxxxxxxx" is
> really just a metadata tag for scripts.
>
> FWIW, I believe Liran no longer works for Oracle, no idea about the others.
>
> > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/pmu.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index edb89b51b383..c896328b2b5a 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -526,12 +526,12 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
> >   	struct kvm_pmc *pmc;
> >   	u64 mask = fast_mode ? ~0u : ~0ull;
> >   
> > -	if (!pmu->version)
> > -		return 1;
> > -
> >   	if (is_vmware_backdoor_pmc(idx))
> >   		return kvm_pmu_rdpmc_vmware(vcpu, idx, data);
> >   
> > +	if (!pmu->version)
> > +		return 1;
> > +
> >   	pmc = static_call(kvm_x86_pmu_rdpmc_ecx_to_pmc)(vcpu, idx, &mask);
> >   	if (!pmc)
> >   		return 1;
> >
> > base-commit: fff2e47e6c3b8050ca26656693caa857e3a8b740
> > --
> > 2.41.0