Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:7 as permitted sender) client-ip=2620:137:e000::3:7;
Message-ID: <93dc0a7aafab5d92bd4ab9b4c1c0622a088bab04.camel@infradead.org>
Subject: Re: [PATCH v4] KVM: x86/tsc: Don't sync user changes to TSC with
 KVM-initiated change
From:   David Woodhouse <dwmw2@infradead.org>
To:     Like Xu <like.xu.linux@gmail.com>
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>
Date:   Wed, 13 Sep 2023 10:44:52 +0200
In-Reply-To: <55dc9282-56c1-8574-0ba1-4bbf075f4c3e@gmail.com>
References: <20230801034524.64007-1-likexu@tencent.com>
         <ZNa9QyRmuAjNAonC@google.com>
         <d39a25a85750e99e11f6b8a58dcd0560f5463f97.camel@infradead.org>
         <55dc9282-56c1-8574-0ba1-4bbf075f4c3e@gmail.com>
Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature";
        boundary="=-cC2nMUc9VkPSGf8aKK51"
User-Agent: Evolution 3.44.4-0ubuntu2 
MIME-Version: 1.0
Precedence: bulk


--=-cC2nMUc9VkPSGf8aKK51
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Wed, 2023-09-13 at 16:41 +0800, Like Xu wrote:
> 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 ke=
rnel and
> > > user-initiated writes, because parts of the code *do* differentiate.=
=C2=A0 I think it's
> > > more accurate to say that the problem is that the sync code doesn't d=
ifferentiate
> > > between userspace initializing the TSC and userspace attempting to sy=
nchronize the
> > > TSC.
> >=20
> > I'm not utterly sure that *I* differentiate between userspace
> > "initializing the TSC" and attempting to "synchronize the TSC". What
> > *is* the difference?
>=20
> I'd be more inclined to Oliver's explanation in this version of the chang=
elog
> 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 synchronizat=
ion is not
> expected for guest based on user's bugzilla report.
>=20

Yes, it's about the kernel's default startup values (first vCPU
starting at TSC 0, others syncing to that on creation), and the fact
that the *first* userspace write (to any vCPU) should actually be
honoured even if it *does* happen to be within 1 second of the kernel's
startup values.


> 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.

I'm slightly dubious about making *changes* to an established userspace
ABI, especially when there's already a better way to do it. But I
suppose this specific change, if you *don't* also take away the ability
for userspace to explicitly write zero to force a sync (qv), is OK.

--=-cC2nMUc9VkPSGf8aKK51
Content-Type: application/pkcs7-signature; name="smime.p7s"
Content-Disposition: attachment; filename="smime.p7s"
Content-Transfer-Encoding: base64
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--=-cC2nMUc9VkPSGf8aKK51--