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From: Oliver Upton To: Shaoqin Huang Cc: Marc Zyngier , kvmarm@lists.linux.dev, Eric Auger , Sebastian Ott , Cornelia Huck , Catalin Marinas , James Morse , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Paolo Bonzini , Shuah Khan , Suzuki K Poulose , Will Deacon , Zenghui Yu Subject: Re: [RFC PATCH v1 0/2] KVM: arm64: Making BT Field in ID_AA64PFR1_EL1 writable Message-ID: References: <20240612023553.127813-1-shahuang@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240612023553.127813-1-shahuang@redhat.com> X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240611_223102_839959_5939C7FA X-CRM114-Status: GOOD ( 12.98 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Shaoqin, On Tue, Jun 11, 2024 at 10:35:50PM -0400, Shaoqin Huang wrote: > Hi guys, > > I'm trying to enable migration from MtCollins(Ampere Altra, ARMv8.2+) to > AmpereOne(AmpereOne, ARMv8.6+), the migration always fails when migration from > MtCollins to AmpereOne due to some register fields differing between the > two machines. > > In this patch series, we try to make more register fields writable like > ID_AA64PFR1_EL1.BT. This is first step towards making the migration possible. > Some other hurdles need to be overcome. This is not sufficient to make the > migration successful from MtCollins to AmpereOne. It isn't possible to transparently migrate between these systems. The former has a cntfrq of 25MHz, and the latter has a cntfrq of 1GHz. There isn't a mechanism for scaling the counter frequency, and I have zero appetite for a paravirt interface. On top of that, erratum AC03_CPU_38 seems to make a migration from Neoverse-N1 to AmpereOne quite perilous, unless you hide FEAT_HAFDBS on the source. These issues are separate, though, from any possible changes to the writability of ID_AA64PFR1_EL1, which still may be useful to userspace. -- Thanks, Oliver