From: Jim Mattson <jmattson@google.com>
To: Eduardo Habkost <ehabkost@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>, Jonathan Corbet <corbet@lwn.net>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
"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>,
Vlastimil Babka <vbabka@suse.cz>, Tony Luck <tony.luck@intel.com>,
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Kyung Min Park <kyung.min.park@intel.com>,
Fenghua Yu <fenghua.yu@intel.com>,
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Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
Brijesh Singh <brijesh.singh@amd.com>,
Dave Hansen <dave.hansen@intel.com>,
Mike Rapoport <rppt@kernel.org>,
Anthony Steinhauser <asteinhauser@google.com>,
Anand K Mistry <amistry@google.com>,
Andi Kleen <ak@linux.intel.com>, Miguel Ojeda <ojeda@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Joe Perches <joe@perches.com>,
linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-perf-users@vger.kernel.org, kvm@vger.kernel.org,
Jiri Denemark <jdenemar@redhat.com>,
"libvir-list@redhat.com" <libvir-list@redhat.com>,
Michal Privoznik <mprivozn@redhat.com>
Subject: Re: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE
Date: Wed, 7 Jul 2021 09:42:26 -0700 [thread overview]
Message-ID: <CALMp9eSJyvU1=FndZyR+hZMtKPWwgibKisBqp0Xcx4jxjrWn2w@mail.gmail.com> (raw)
In-Reply-To: <CAOpTY_paTO=xqfGXPCC2Paty5ptJ6Dqpo4Tzb4C2hrO_t=vS8w@mail.gmail.com>
On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost@redhat.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@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
>
next prev parent reply other threads:[~2021-07-07 16:42 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-09 20:57 [PATCH 0/4] TSX force abort Pawan Gupta
2021-06-09 20:58 ` [PATCH 1/4] x86/msr: Define new bits in TSX_FORCE_ABORT MSR Pawan Gupta
2021-06-11 8:39 ` Borislav Petkov
2021-06-11 21:31 ` Pawan Gupta
2021-06-09 21:12 ` [PATCH 2/4] perf/x86/intel: Do not deploy workaround when TSX is deprecated Pawan Gupta
2021-06-11 7:50 ` Borislav Petkov
2021-06-11 21:34 ` Pawan Gupta
2021-06-11 22:01 ` Borislav Petkov
2021-06-11 23:21 ` Pawan Gupta
2021-06-09 21:13 ` [PATCH 3/4] x86/tsx: Clear CPUID bits when TSX always force aborts Pawan Gupta
2021-06-11 10:03 ` Borislav Petkov
2021-06-11 21:36 ` Pawan Gupta
2021-06-09 21:14 ` [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE Pawan Gupta
2021-06-11 10:06 ` Borislav Petkov
2021-06-11 21:37 ` Pawan Gupta
2021-07-06 19:52 ` Eduardo Habkost
2021-07-06 21:05 ` Paolo Bonzini
2021-07-06 21:33 ` Eduardo Habkost
2021-07-06 21:58 ` Paolo Bonzini
2021-07-07 15:08 ` Eduardo Habkost
2021-07-07 16:42 ` Jim Mattson [this message]
2021-07-07 17:08 ` Eduardo Habkost
2021-07-07 17:15 ` Jim Mattson
2021-07-07 18:23 ` Eduardo Habkost
2021-07-08 9:15 ` Paolo Bonzini
2021-07-06 21:16 ` Pawan Gupta
2021-07-06 21:19 ` Eduardo Habkost
2021-07-06 22:51 ` Pawan Gupta
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