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From: Oliver Upton To: "Russell King (Oracle)" Cc: Jianyong Wu , maz@kernel.org, james.morse@arm.com, will@kernel.org, salil.mehta@huawei.com, suzuki.poulose@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, justin.he@arm.com Subject: Re: [PATCH] arm64/kvm: Introduce feature extension for SMCCC filter Message-ID: References: <20231116114152.912344-1-jianyong.wu@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231116_152238_930847_9CF67FE4 X-CRM114-Status: GOOD ( 18.86 ) 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 On Thu, Nov 16, 2023 at 07:06:18PM +0000, Russell King (Oracle) wrote: > On Thu, Nov 16, 2023 at 11:41:52AM +0000, Jianyong Wu wrote: > > 821d935c87b introduces support for userspace SMCCC filtering, but lack > > of a way to tell userspace if we have this feature. Add a corresponding > > feature extension can resolve this issue. > > > > For example, the incoming feature Vcpu Hotplug needs the SMCCC filter. > > As there is no way to check this feature, VMM will run into error when > > it calls this feature on an old kernel. It's bad for backward compatible. > > Can't you just attempt to use the SMCCC filtering, and if it errors out > with the appropriate error code, decide that SMCCC filtering is not > available? That would also work, as we return ENXIO for the unsupported ioctl. > That's how most things like kernel syscalls work - if they're not > implemented they return -ENOSYS. glibc can detect that and use a > fallback. I generally agree, but KVM has gone in the other direction of providing auxiliary interfaces for discovering new UAPI. ENXIO has been slightly overloaded to imply that a given ioctl is non-existent or otherwise unsupported due to some dynamic configuration. Is it ideal? Of course not. With that said userspace may as well use the preferred / documented discoverability mechanism. And in Jianyong's case the KVM documentation is rather unambiguous (for once) about how you discover device attributes. https://docs.kernel.org/virt/kvm/api.html#kvm-has-device-attr -- Thanks, Oliver _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel