From: Josh Poimboeuf <jpoimboe@kernel.org>
To: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
Jon Kohler <jon@nutanix.com>,
Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
"Peter Zijlstra (Intel)" <peterz@infradead.org>,
Daniel Sneddon <daniel.sneddon@linux.intel.com>,
"kvm @ vger . kernel . org" <kvm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host()
Date: Wed, 31 May 2023 21:23:45 -0700 [thread overview]
Message-ID: <20230601042345.52s5337uz62p6aow@treble> (raw)
In-Reply-To: <20230601012323.36te7hfv366danpf@desk>
On Wed, May 31, 2023 at 06:24:29PM -0700, Pawan Gupta wrote:
## 2023-05-31
> On Thu, Jun 01, 2023 at 01:50:48AM +0100, Andrew Cooper wrote:
> > On 01/06/2023 1:42 am, Josh Poimboeuf wrote:
> > > So each LFENCE has a distinct purpose. That said, there are no indirect
> > > branches or unbalanced RETs between them.
> >
> > How lucky are you feeling?
> >
> > You're in C at this point, which means the compiler could have emitted a
> > call to mem{cpy,cmp}() in place of a simple assignment/comparison.
>
> Moving the second LFENCE to the else part of WRMSR should be possible?
> So that the serialization can be achived either by WRMSR or LFENCE. This
> saves an LFENCE when host and guest value of MSR_SPEC_CTRL differ.
Yes. Though in practice it might not make much of a difference. With
wrmsr+lfence, the lfence has nothing to do so it might be almost
instantaneous anyway.
--
Josh
next prev parent reply other threads:[~2023-06-01 4:24 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-31 15:01 [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host() Jon Kohler
2023-05-31 23:18 ` Josh Poimboeuf
2023-05-31 23:58 ` Jon Kohler
2023-06-01 0:42 ` Josh Poimboeuf
2023-06-01 0:50 ` Andrew Cooper
2023-06-01 0:56 ` Josh Poimboeuf
2023-06-01 1:24 ` Pawan Gupta
2023-06-01 4:23 ` Josh Poimboeuf [this message]
2023-06-05 14:29 ` Jon Kohler
2023-06-05 16:35 ` Josh Poimboeuf
2023-06-05 16:39 ` Jon Kohler
2023-06-05 17:31 ` Pawan Gupta
2023-06-05 18:31 ` Sean Christopherson
2023-06-05 19:57 ` Jon Kohler
2023-06-05 20:01 ` Josh Poimboeuf
2023-06-06 0:20 ` Andrew Cooper
2023-06-06 3:59 ` Josh Poimboeuf
2023-06-01 0:29 ` Andrew Cooper
2023-06-01 0:53 ` Josh Poimboeuf
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