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From: Oliver Upton To: Cornelia Huck Cc: Jing Zhang , KVM , KVMARM , ARMLinux , Marc Zyngier , Oliver Upton , Will Deacon , Paolo Bonzini , James Morse , Alexandru Elisei , Suzuki K Poulose , Fuad Tabba , Reiji Watanabe , Raghavendra Rao Ananta , Suraj Jitindar Singh Subject: Re: [PATCH v4 1/4] KVM: arm64: Enable writable for ID_AA64DFR0_EL1 Message-ID: References: <20230607194554.87359-1-jingzhangos@google.com> <20230607194554.87359-2-jingzhangos@google.com> <874jmjiumh.fsf@redhat.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <874jmjiumh.fsf@redhat.com> X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230704_090410_783858_BAFB4370 X-CRM114-Status: GOOD ( 29.48 ) 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: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Cornelia, On Tue, Jul 04, 2023 at 05:06:30PM +0200, Cornelia Huck wrote: > On Mon, Jun 26 2023, Oliver Upton wrote: > > > On Wed, Jun 07, 2023 at 07:45:51PM +0000, Jing Zhang wrote: > >> + brps = FIELD_GET(ID_AA64DFR0_EL1_BRPs_MASK, val); > >> + ctx_cmps = FIELD_GET(ID_AA64DFR0_EL1_CTX_CMPs_MASK, val); > >> + if (ctx_cmps > brps) > >> + return -EINVAL; > >> + > > > > I'm not fully convinced on the need to do this sort of cross-field > > validation... I think it is probably more trouble than it is worth. If > > userspace writes something illogical to the register, oh well. All we > > should care about is that the advertised feature set is a subset of > > what's supported by the host. > > > > The series doesn't even do complete sanity checking, and instead works > > on a few cherry-picked examples. AA64PFR0.EL{0-3} would also require > > special handling depending on how pedantic you're feeling. AArch32 > > support at a higher exception level implies AArch32 support at all lower > > exception levels. > > > > But that isn't a suggestion to implement it, more of a suggestion to > > just avoid the problem as a whole. > > Generally speaking, how much effort do we want to invest to prevent > userspace from doing dumb things? "Make sure we advertise a subset of > features of what the host supports" and "disallow writing values that > are not allowed by the architecture in the first place" seem reasonable, > but if userspace wants to create weird frankencpus[1], should it be > allowed to break the guest and get to keep the pieces? What I'm specifically objecting to is having KVM do sanity checks across multiple fields. That requires explicit, per-field plumbing that will eventually become a tangled mess that Marc and I will have to maintain. The context-aware breakpoints is one example, as is ensuring SVE is exposed iff FP is too. In all likelihood we'll either get some part of this wrong, or miss a required check altogether. Modulo a few exceptions to this case, I think per-field validation is going to cover almost everything we're worried about, and we get that largely for free from arm64_check_features(). > I'd be more in favour to rely on userspace to configure something that > is actually usable; it needs to sanitize any user-provided configuration > anyway. Just want to make sure I understand your sentiment here, you'd be in favor of the more robust sanitization? > [1] I think userspace will end up creating frankencpus in any case, but > at least it should be the kind that doesn't look out of place in the > subway if you dress it in proper clothing. I mean, KVM already advertises a frankencpu in the first place, so we're off to a good start :) -- Thanks, Oliver _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel