linux-kernel - Re: [PATCH v4 14/19] KVM: SVM: Check that the current CPU supports SVM in kvm_is_svm

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Message-ID: <ZMKd2cT2fw4ZiJQp@google.com>
Date:   Thu, 27 Jul 2023 09:39:53 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Kai Huang <kai.huang@...el.com>, Chao Gao <chao.gao@...el.com>
Subject: Re: [PATCH v4 14/19] KVM: SVM: Check that the current CPU supports
 SVM in kvm_is_svm_supported()

On Tue, Jul 25, 2023, Peter Zijlstra wrote:
> On Mon, Jul 24, 2023 at 02:40:03PM -0700, Sean Christopherson wrote:
> > On Mon, Jul 24, 2023, Peter Zijlstra wrote:
> > > On Fri, Jul 21, 2023 at 01:18:54PM -0700, 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.
> > > 
> > > Is that a realistic concern?
> > 
> > It's not a concern in the sense that it should never happen, but I know of at
> > least one example where VMX on Intel completely disappeared[1].  The "compatibility"
> > checks are really more about the entire VMX/SVM feature set, the base VMX/SVM
> > support check is just an easy and obvious precursor to the full compatibility
> > checks.
> > 
> > Of course, SVM doesn't currently have compatibility checks on the full SVM feature
> > set, but that's more due to lack of a forcing function than a desire to _not_ have
> > them.  Intel CPUs have a pesky habit of bugs, ucode updates, and/or in-field errors
> > resulting in VMX features randomly appearing or disappearing.  E.g. there's an
> > ongoing buzilla (sorry) issue[2] where a user is only able to load KVM *after* a
> > suspend+resume cycle, because TSC scaling only shows up on one socket immediately
> > after boot, which is then somehow resolved by suspend+resume.
> > 
> > [1] 009bce1df0bb ("x86/split_lock: Don't write MSR_TEST_CTRL on CPUs that aren't whitelisted")
> > [2] https://bugzilla.kernel.org/show_bug.cgi?id=217574
> 
> Is that using late loading of ucode?

Not sure, though I don't think that is relevant for this particular bug.

> Anything that changes *any* feature flag must be early ucode load, there is
> no other possible way since einux does feature enumeration early, and
> features are fixed thereafter.
> 
> This is one of the many reasons late loading is a trainwreck.
> 
> Doing suspend/resume probably re-loads the firmware

Ya, it does.

> and re-does the feature enumeration -- I didn't check.

The reported ucode revision is the same before and after resume, and is consistent
across all CPUs.  KVM does the per-CPU feature enumeration (for sanity checks)
everytime userspace attempts to load KVM (the module), so the timing of the ucode
patch load _shouldn't_ matter.

The user is running quite old ucode for their system, so the current theory is that
old buggy ucode is to blame.