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From:   Eduardo Habkost <ehabkost@redhat.com>
Date:   Wed, 7 Jul 2021 11:08:32 -0400
Message-ID: <CAOpTY_paTO=xqfGXPCC2Paty5ptJ6Dqpo4Tzb4C2hrO_t=vS8w@mail.gmail.com>
Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits
 RTM and HLE
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
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        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
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        Namhyung Kim <namhyung@kernel.org>, x86@kernel.org,
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        "Paul E. McKenney" <paulmck@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Viresh Kumar <viresh.kumar@linaro.org>,
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        kvm@vger.kernel.org, Jiri Denemark <jdenemar@redhat.com>,
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        Michal Privoznik <mprivozn@redhat.com>
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Precedence: bulk

CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
that the definition of "supported CPU features" will probably become a
bit more complex in the future.

On Tue, Jul 6, 2021 at 5:58 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 06/07/21 23:33, Eduardo Habkost wrote:
> > On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >> It's a bit tricky, because HLE and RTM won't really behave well.  An old
> >> guest that sees RTM=1 might end up retrying and aborting transactions
> >> too much.  So I'm not sure that a QEMU "-cpu host" guest should have HLE
> >> and RTM enabled.
> >
> > Is the purpose of GET_SUPPORTED_CPUID to return what is supported by
> > KVM, or to return what "-cpu host" should enable by default? They are
> > conflicting requirements in this case.
>
> In theory there is GET_EMULATED_CPUID for the former, so it should be
> the latter.  In practice neither QEMU nor Libvirt use it; maybe now we
> have a good reason to add it, but note that userspace could also check
> host RTM_ALWAYS_ABORT.
>
> > Returning HLE=1,RTM=1 in GET_SUPPORTED_CPUID makes existing userspace
> > take bad decisions until it's updated.
> >
> > Returning HLE=0,RTM=0 in GET_SUPPORTED_CPUID prevents existing
> > userspace from resuming existing VMs (despite being technically
> > possible).
> >
> > The first option has an easy workaround that doesn't require a
> > software update (disabling HLE/RTM in the VM configuration). The
> > second option doesn't have a workaround. I'm inclined towards the
> > first option.
>
> The default has already been tsx=off for a while though, so checking
> either GET_EMULATED_CPUID or host RTM_ALWAYS_ABORT in userspace might
> also be feasible for those that are still on tsx=on.

This sounds like a perfect use case for GET_EMULATED_CPUID. My only
concern is breaking existing userspace.

But if this was already broken for a few kernel releases due to
tsx=off being the default, maybe GET_EMULATED_CPUID will be a
reasonable approach.

--
Eduardo