From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta1.migadu.com (out-172.mta1.migadu.com [95.215.58.172]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 89CE82FE26 for ; Thu, 16 Nov 2023 23:22:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="R8N+dwXM" Date: Thu, 16 Nov 2023 15:22:10 -0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1700176949; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ql19/OGj64AFFaCoG3rNb0IvzQY8kBjJxvcnLCSOPkE=; b=R8N+dwXMjjEbQXwnXgnMViFOkdYDxySMg7onZbQpLj/tOmnHLechypeTzAq+V0LetrVLWJ WA5CT9AJKg+AwMhnccXO7mdC0rgkOjk+G+5eyWGxMT2HDH7UMds2PWp178gFqUDSq6S4z2 pFHr6VXLWTQNKBE3e33gvmO+YXukj8g= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. 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> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT 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