linux-kernel - Re: [PATCH v2 14/18] KVM: SVM: Check that the current CPU supports SVM in kvm_is_svm

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Date:   Mon, 13 Mar 2023 10:29:58 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Chao Gao <chao.gao@...el.com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 14/18] KVM: SVM: Check that the current CPU supports
 SVM in kvm_is_svm_supported()

On Mon, Mar 13, 2023, Huang, Kai wrote:
> On Fri, 2023-03-10 at 13:42 -0800, Sean Christopherson wrote:
> > Check "this" CPU instead of the boot CPU when querying SVM support so that
> > the per-CPU checks done during hardware enabling actually function as
> > intended, i.e. will detect issues where SVM isn't support on all CPUs.
> > 
> > Disable migration for the use from svm_init() mostly so that the standard
> > accessors for the per-CPU data can be used without getting yelled at by
> > CONFIG_DEBUG_PREEMPT=y sanity checks.  Preventing the "disabled by BIOS"
> > error message from reporting the wrong CPU is largely a bonus, as ensuring
> > a stable CPU during module load is a non-goal for KVM.
> > 
> > Link: https://lore.kernel.org/all/ZAdxNgv0M6P63odE@google.com
> > Cc: Kai Huang <kai.huang@...el.com>
> > Cc: Chao Gao <chao.gao@...el.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> 
> Should we add:
> 
> Fixes: c82a5c5c53c5 ("KVM: x86: Do compatibility checks when onlining CPU")
> 
> As that commit introduced using raw_smp_processor_id() to get CPU id in
> kvm_is_svm_supported() and print the CPU id out in error message?

My vote is to not to add a Fixes because using raw_smp_processor_id() and not disabling
migration for module probe case was deliberate and is safe.  I don't want to give the
impression that the existing code is functionally broken.  The only quirk is that
the reporting could be misleading.

That said, I'm not against adding a Fixes tag, because I certainly can't argue
against the reporting being flawed.

> > ---
> >  arch/x86/kvm/svm/svm.c | 25 +++++++++++++++++++------
> >  1 file changed, 19 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index 2934f185960d..f04b61c3d9d8 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -520,18 +520,20 @@ static void svm_init_osvw(struct kvm_vcpu *vcpu)
> >  		vcpu->arch.osvw.status |= 1;
> >  }
> >  
> > -static bool kvm_is_svm_supported(void)
> > +static bool __kvm_is_svm_supported(void)
> >  {
> > -	int cpu = raw_smp_processor_id();
> > +	int cpu = smp_processor_id();
> 
> Since we have made sure __kvm_is_svm_supported() is always performed on a stable
> cpu, should we keep using raw_smp_processor_id()? ï¿½
> 
> It is faster than smp_processor_id() when CONFIG_DEBUG_PREEMPT=y, but yes the
> latter can help to catch bug.

Most kernels with any amount of CONFIG_DEBUG_* options enabled are comically slow
anyways, I much prefer having the sanity checks than the performance.