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Fri, 06 Aug 2021 03:32:45 -0700 (PDT) Received: from localhost ([118.210.97.79]) by smtp.gmail.com with ESMTPSA id 20sm12630150pgg.36.2021.08.06.03.32.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Aug 2021 03:32:45 -0700 (PDT) Date: Fri, 06 Aug 2021 20:32:40 +1000 From: Nicholas Piggin Subject: Re: [PATCH v1 14/55] KVM: PPC: Book3S HV: Don't always save PMU for guest capable of nesting To: kvm-ppc@vger.kernel.org, Michael Ellerman References: <20210726035036.739609-1-npiggin@gmail.com> <20210726035036.739609-15-npiggin@gmail.com> <871r77ni1g.fsf@mpe.ellerman.id.au> In-Reply-To: <871r77ni1g.fsf@mpe.ellerman.id.au> MIME-Version: 1.0 Message-Id: <1628245856.8cocc7zj8u.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Excerpts from Michael Ellerman's message of August 6, 2021 5:34 pm: > Nicholas Piggin writes: >> Revert the workaround added by commit 63279eeb7f93a ("KVM: PPC: Book3S >> HV: Always save guest pmu for guest capable of nesting"). >> >> Nested capable guests running with the earlier commit ("KVM: PPC: Book3S >> HV Nested: Indicate guest PMU in-use in VPA") will now indicate the PMU >> in-use status of their guests, which means the parent does not need to >> unconditionally save the PMU for nested capable guests. >> >> This will cause the PMU to break for nested guests when running older >> nested hypervisor guests under a kernel with this change. It's unclear >> there's an easy way to avoid that, so this could wait for a release or >> so for the fix to filter into stable kernels. >=20 > I'm not sure PMU inside nested guests is getting much use, but I don't > think we can break this quite so casually :) >=20 > Especially as the failure mode will be PMU counts that don't match > reality, which is hard to diagnose. It took nearly a year for us to find > the original bug. >=20 > I think we need to hold this back for a while. >=20 > We could put it under a CONFIG option, and then flip that option to off > at some point in the future. Okay that might be a good compromise, I'll do that. Thanks, Nick