From: Like Xu <like.xu.linux@gmail.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Oliver Upton <oliver.upton@linux.dev>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Sean Christopherson <seanjc@google.com>
Subject: Re: [PATCH v4] KVM: x86/tsc: Don't sync user changes to TSC with KVM-initiated change
Date: Wed, 13 Sep 2023 16:41:07 +0800 [thread overview]
Message-ID: <55dc9282-56c1-8574-0ba1-4bbf075f4c3e@gmail.com> (raw)
In-Reply-To: <d39a25a85750e99e11f6b8a58dcd0560f5463f97.camel@infradead.org>
On 13/9/2023 4:10 pm, David Woodhouse wrote:
> On Fri, 2023-08-11 at 15:59 -0700, Sean Christopherson wrote:
>> The problem isn't that the sync code doesn't differentiate between kernel and
>> user-initiated writes, because parts of the code *do* differentiate. I think it's
>> more accurate to say that the problem is that the sync code doesn't differentiate
>> between userspace initializing the TSC and userspace attempting to synchronize the
>> TSC.
>
> I'm not utterly sure that *I* differentiate between userspace
> "initializing the TSC" and attempting to "synchronize the TSC". What
> *is* the difference?
I'd be more inclined to Oliver's explanation in this version of the changelog
that different tsc_offsets are used to calculate guest_tsc value between the vcpu
is created and when it is first set by usersapce. This extra synchronization is not
expected for guest based on user's bugzilla report.
>
> Userspace is merely *setting* the TSC for a given vCPU, regardless of
> whether other vCPUs even exist.
>
> But we have to work around the fundamental brokenness of the legacy
> API, whose semantics are most accurately described as "Please set the
> TSC to precisely <x> because that's what it should have been *some*
> time around now, if I wasn't preempted very much between when I
> calculated it and when you see this ioctl".
>
> That's why — for the legacy API only — we have this hack to make the
> TSCs *actually* in sync if they're close. Because without it, there;s
> *no* way the VMM can restore a guest with its TSCs actually in sync.
>
> I think the best answer to the bug report that led to this patch is
> just "Don't use the legacy API then". Use KVM_VCPU_TSC_OFFSET which is
> defined as "the TSC was <x> at KVM time <y>" and is actually *sane*.
>
Two hands in favor. Using the new KVM_VCPU_TSC_OFFSET API and a little
fix on the legacy API is not conflict. Thank you for reviewing it.
next prev parent reply other threads:[~2023-09-13 8:41 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-01 3:45 [PATCH v4] KVM: x86/tsc: Don't sync user changes to TSC with KVM-initiated change Like Xu
2023-08-11 22:59 ` Sean Christopherson
2023-09-13 8:10 ` David Woodhouse
2023-09-13 8:41 ` Like Xu [this message]
2023-09-13 8:44 ` David Woodhouse
2023-09-13 8:31 ` David Woodhouse
2023-09-13 14:50 ` Sean Christopherson
2023-09-13 15:05 ` David Woodhouse
2023-09-13 15:15 ` Sean Christopherson
2023-09-13 15:24 ` David Woodhouse
2023-09-14 3:24 ` Like Xu
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