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From: Borislav Petkov <bp@alien8.de>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Eduardo Habkost" <ehabkost@redhat.com>,
	"Jörg Rödel" <joro@8bytes.org>,
	"Andre Przywara" <andre.przywara@arm.com>,
	"kvm ML" <kvm@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	"Tom Lendacky" <thomas.lendacky@amd.com>
Subject: Re: kvm: RDTSCP on AMD
Date: Mon, 10 Dec 2018 19:13:28 +0100	[thread overview]
Message-ID: <20181210181328.GA762@zn.tnic> (raw)

Reviving an old thread here.

On Wed, Jul 06, 2016 at 11:27:16PM +0200, Paolo Bonzini wrote:
> On 06/07/2016 19:34, Eduardo Habkost wrote:
> >> > Nothing is needed in the kernel actually.  You can skip the intercept
> >> > by running the guest with MSR_TSC_AUX set to the guest's expected value.
> >> >  Which KVM does, except that it's botched so I need to apply the
> >> > patch in https://lkml.org/lkml/2016/4/13/802.
> > Do you mean -cpu Opteron_G*,+rdtscp will be buggy on Linux v4.5?
> > (v4.5 reports rdtscp as supported in GET_SUPPORTED_CPUID)
> > 
> > Can we do something to make QEMU detect the buggy kernel before
> > allowing rdtscp to be enabled, or should we just tell people to
> > upgrade their kernel?
> 
> We usually just tell people to use the latest stable kernel.
> 
> Adding new CPU models is not a big deal, in fact it's almost easier than
> getting compat properties right. :)

Ok, can we finally revert

  33b5e8c03ae7 ("target-i386: Disable rdtscp on Opteron_G* CPU models")

in the qemu tree?

Three years should be enough by now for

  46896c73c1a4 ("KVM: svm: add support for RDTSCP")

to have percolated downstream.

Btw, its commit message talks about Linux not using RDTSCP but that will
change soon. :-)

Oh, and the EPYC qemu CPU model has CPUID_EXT2_RDTSCP and works just
fine when patching in RDTSCP:

[    0.543197] apply_alternatives: feat: 3*32+18, old: (read_tsc+0x0/0x10 (ffffffff8101d1c0) len: 5), repl: (ffffffff824e6d33, len: 5), pad: 3
[    0.544448] ffffffff8101d1c0: old_insn: 0f 31 90 90 90
[    0.545023] ffffffff824e6d33: rpl_insn: 0f ae e8 0f 31
[    0.545598] ffffffff8101d1c0: final_insn: 0f ae e8 0f 31
[    0.546193] apply_alternatives: feat: 1*32+27, old: (read_tsc+0x0/0x10 (ffffffff8101d1c0) len: 5), repl: (ffffffff824e6d38, len: 3), pad: 3
[    0.547195] ffffffff8101d1c0: old_insn: 0f ae e8 0f 31
[    0.547775] ffffffff824e6d38: rpl_insn: 0f 01 f9
[    0.548307] ffffffff8101d1c0: final_insn: 0f 01 f9 66 90

That final_insn which gets patched to by the alternatives is

ffffffff8101d1c0:       0f 01 f9                rdtscp
ffffffff8101d1c3:       66 90                   xchg %ax,%ax

Thx.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

             reply	other threads:[~2018-12-10 18:13 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-10 18:13 Borislav Petkov [this message]
2018-12-10 18:37 ` kvm: RDTSCP on AMD Eduardo Habkost
2018-12-10 18:41   ` Borislav Petkov
2018-12-10 19:06     ` Eduardo Habkost
2018-12-10 19:42       ` Borislav Petkov
2018-12-10 20:08         ` Eduardo Habkost
2018-12-10 20:39           ` Borislav Petkov
2018-12-11 10:38           ` Daniel P. Berrangé
2018-12-11 11:55             ` Eduardo Habkost
2018-12-11 14:35               ` Daniel P. Berrangé
2018-12-11 15:23               ` Paolo Bonzini
2018-12-11 15:30                 ` Daniel P. Berrangé
2018-12-11 16:14                   ` [PATCH] target-i386: Reenable RDTSCP support on Opteron_G[345] CPU models Borislav Petkov
2018-12-11 16:14                     ` [Qemu-devel] " Borislav Petkov
2018-12-12 19:52                     ` Eduardo Habkost
2018-12-12 19:52                       ` [Qemu-devel] " Eduardo Habkost
2018-12-12 20:08                       ` [PATCH -v2] target-i386: Reenable RDTSCP support on Opteron_G[345] CPU models " Borislav Petkov
2018-12-12 20:08                         ` [Qemu-devel] " Borislav Petkov
2018-12-20 12:10                         ` Eduardo Habkost
2018-12-20 12:10                           ` [Qemu-devel] " Eduardo Habkost
2018-12-10 19:47     ` kvm: RDTSCP on AMD Eduardo Habkost
2018-12-10 20:16       ` Borislav Petkov
  -- strict thread matches above, loose matches on Subject: below --
2016-07-06 12:44 Borislav Petkov
2016-07-06 13:01 ` Paolo Bonzini
2016-07-06 17:34   ` Eduardo Habkost
2016-07-06 21:27     ` Paolo Bonzini

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