linux-kernel - Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE

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Message-ID: <671be35f-220a-f583-aa31-3a2da7dae93a@redhat.com>
Date:   Tue, 6 Jul 2021 23:58:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Jonathan Corbet <corbet@....net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Viresh Kumar <viresh.kumar@...aro.org>,
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        Fenghua Yu <fenghua.yu@...el.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
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        Mike Rapoport <rppt@...nel.org>,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        Anand K Mistry <amistry@...gle.com>,
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        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits
 RTM and HLE

On 06/07/21 23:33, Eduardo Habkost wrote:
> On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini@...hat.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.

Paolo