From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
jmattson@google.com, stable@vger.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even if tsx=off
Date: Mon, 1 Feb 2021 18:34:34 +0100 [thread overview]
Message-ID: <cac389ad-96b0-293e-f977-4e9c6d719dea@redhat.com> (raw)
In-Reply-To: <YBgugM03fsEiOxz1@google.com>
On 01/02/21 17:38, Sean Christopherson wrote:
>>>> /*
>>>> * On TAA affected systems:
>>>> * - nothing to do if TSX is disabled on the host.
>>>> * - we emulate TSX_CTRL if present on the host.
>>>> * This lets the guest use VERW to clear CPU buffers.
>>>> */
>
> it says "nothing to do..." and then clears a
> flag. The other interpretation of "nothing to do... at runtime" is also wrong
> as KVM emulates the MSR as a nop.
>
> I guess I just find the whole comment more confusing than the code itself.
What about:
if (!boot_cpu_has(X86_FEATURE_RTM)) {
/*
* If RTM=0 because the kernel has disabled TSX, the
host might
* have TAA_NO or TSX_CTRL. Clear TAA_NO (the guest
sees RTM=0
* and therefore knows that there cannot be TAA) but keep
* TSX_CTRL: some buggy userspaces leave it set on
tsx=on hosts,
* and we want to allow migrating those guests to
tsx=off hosts.
*/
data &= ~ARCH_CAP_TAA_NO;
} else if (!boot_cpu_has_bug(X86_BUG_TAA)) {
data |= ARCH_CAP_TAA_NO;
} else {
/*
* Nothing to do here; we emulate TSX_CTRL if present
on the
* host so the guest can choose between disabling TSX or
* using VERW to clear CPU buffers.
*/
}
Paolo
next prev parent reply other threads:[~2021-02-01 17:36 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-29 10:19 [PATCH v2] KVM: x86: Allow guests to see MSR_IA32_TSX_CTRL even if tsx=off Paolo Bonzini
2021-01-29 16:58 ` Sean Christopherson
2021-02-01 8:46 ` Paolo Bonzini
2021-02-01 9:08 ` Paolo Bonzini
2021-02-01 16:38 ` Sean Christopherson
2021-02-01 17:34 ` Paolo Bonzini [this message]
2021-02-01 17:36 ` Sean Christopherson
2021-02-01 17:03 ` Sean Christopherson
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