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

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Date:   Wed, 7 Jul 2021 09:42:26 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tony Luck <tony.luck@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Kyung Min Park <kyung.min.park@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Krish Sadhukhan <krish.sadhukhan@...cle.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Joerg Roedel <jroedel@...e.de>,
        Victor Ding <victording@...gle.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Dave Hansen <dave.hansen@...el.com>,
        Mike Rapoport <rppt@...nel.org>,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        Anand K Mistry <amistry@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joe Perches <joe@...ches.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        kvm@...r.kernel.org, Jiri Denemark <jdenemar@...hat.com>,
        "libvir-list@...hat.com" <libvir-list@...hat.com>,
        Michal Privoznik <mprivozn@...hat.com>
Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits
 RTM and HLE

On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost@...hat.com> wrote:
>
> 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.

Has there ever been a clear definition? Family, model, and stepping,
for instance: are these the only values supported? That would make
cross-platform migration impossible. What about the vendor string? Is
that the only value supported? That would make cross-vendor migration
impossible. For the maximum input value for basic CPUID information
(CPUID.0H:EAX), is that the only value supported, or is it the maximum
value supported? On the various individual feature bits, does a '1'
imply that '0' is also supported, or is '1' the only value supported?
What about the feature bits with reversed polarity (e.g.
CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])?

This API has never made sense to me. I have no idea how to interpret
what it is telling me.

> On Tue, Jul 6, 2021 at 5:58 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >
> > 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.
>
> 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
>