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[23.128.96.18]) by mx.google.com with ESMTP id p13si7273588ejx.702.2020.11.09.07.19.45; Mon, 09 Nov 2020 07:20:09 -0800 (PST) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@redhat.com header.s=mimecast20190719 header.b="Hw5/cdlN"; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730637AbgKIPST (ORCPT + 99 others); Mon, 9 Nov 2020 10:18:19 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:23560 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729868AbgKIPSS (ORCPT ); Mon, 9 Nov 2020 10:18:18 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1604935097; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=w5enKsZSNWbgWd9sqRj4xxK6HKbx1Pcury6gxzpl2VM=; b=Hw5/cdlNaz6X3pipgVMyX3SCd1YLq3Ibi9D3362pwpWCTppK7mb52xqwdAD2gtC8ZnPzMb LI2DBPp6ZtHRdvxbYLkX6Clr4kz1LoPKosIyxKyWWOSLMticl9VS79oRCDrc2V8CNT1IFv bEa3Otp4IYE8qRJ2tboZEZM1nXtj6tI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-326-xfMPrt9PP7uOHDkprBj0xg-1; Mon, 09 Nov 2020 10:18:15 -0500 X-MC-Unique: xfMPrt9PP7uOHDkprBj0xg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 84ADA876E3D; Mon, 9 Nov 2020 15:18:14 +0000 (UTC) Received: from [10.36.114.125] (ovpn-114-125.ams2.redhat.com [10.36.114.125]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6F50C60BF1; Mon, 9 Nov 2020 15:18:09 +0000 (UTC) Subject: Re: [RFC, v0 1/3] vfio/platform: add support for msi To: Vikas Gupta , Alex Williamson Cc: Cornelia Huck , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Vikram Prakash References: <20201105060257.35269-1-vikas.gupta@broadcom.com> <20201105060257.35269-2-vikas.gupta@broadcom.com> <20201105000806.1df16656@x1.home> <20201105201208.5366d71e@x1.home> From: Auger Eric Message-ID: Date: Mon, 9 Nov 2020 16:18:07 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Vikas, On 11/9/20 7:41 AM, Vikas Gupta wrote: > Hi Alex, > > On Fri, Nov 6, 2020 at 8:42 AM Alex Williamson > wrote: >> >> On Fri, 6 Nov 2020 08:24:26 +0530 >> Vikas Gupta wrote: >> >>> Hi Alex, >>> >>> On Thu, Nov 5, 2020 at 12:38 PM Alex Williamson >>> wrote: >>>> >>>> On Thu, 5 Nov 2020 11:32:55 +0530 >>>> Vikas Gupta wrote: >>>> >>>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h >>>>> index 2f313a238a8f..aab051e8338d 100644 >>>>> --- a/include/uapi/linux/vfio.h >>>>> +++ b/include/uapi/linux/vfio.h >>>>> @@ -203,6 +203,7 @@ struct vfio_device_info { >>>>> #define VFIO_DEVICE_FLAGS_AP (1 << 5) /* vfio-ap device */ >>>>> #define VFIO_DEVICE_FLAGS_FSL_MC (1 << 6) /* vfio-fsl-mc device */ >>>>> #define VFIO_DEVICE_FLAGS_CAPS (1 << 7) /* Info supports caps */ >>>>> +#define VFIO_DEVICE_FLAGS_MSI (1 << 8) /* Device supports msi */ >>>>> __u32 num_regions; /* Max region index + 1 */ >>>>> __u32 num_irqs; /* Max IRQ index + 1 */ >>>>> __u32 cap_offset; /* Offset within info struct of first cap */ >>>> >>>> This doesn't make any sense to me, MSIs are just edge triggered >>>> interrupts to userspace, so why isn't this fully described via >>>> VFIO_DEVICE_GET_IRQ_INFO? If we do need something new to describe it, >>>> this seems incomplete, which indexes are MSI (IRQ_INFO can describe >>>> that)? We also already support MSI with vfio-pci, so a global flag for >>>> the device advertising this still seems wrong. Thanks, >>>> >>>> Alex >>>> >>> Since VFIO platform uses indexes for IRQ numbers so I think MSI(s) >>> cannot be described using indexes. >> >> That would be news for vfio-pci which has been describing MSIs with >> sub-indexes within indexes since vfio started. >> >>> In the patch set there is no difference between MSI and normal >>> interrupt for VFIO_DEVICE_GET_IRQ_INFO. >> >> Then what exactly is a global device flag indicating? Does it indicate >> all IRQs are MSI? > > No, it's not indicating that all are MSI. > The rationale behind adding the flag to tell user-space that platform > device supports MSI as well. As you mentioned recently added > capabilities can help on this, I`ll go through that. > >> >>> The patch set adds MSI(s), say as an extension, to the normal >>> interrupts and handled accordingly. >> >> So we have both "normal" IRQs and MSIs? How does the user know which >> indexes are which? > > With this patch set, I think this is missing and user space cannot > know that particular index is MSI interrupt. > For platform devices there is no such mechanism, like index and > sub-indexes to differentiate between legacy, MSI or MSIX as it’s there > in PCI. Wht can't you use the count field (as per vfio_pci_get_irq_count())? > I believe for a particular IRQ index if the flag > VFIO_IRQ_INFO_NORESIZE is used then user space can know which IRQ > index has MSI(s). Does it make sense? I don't think it is the same semantics. Thanks Eric > Suggestions on this would be helpful. > > Thanks, > Vikas >> >>> Do you see this is a violation? If >> >> Seems pretty unclear and dubious use of a global device flag. >> >>> yes, then we`ll think of other possible ways to support MSI for the >>> platform devices. >>> Macro VFIO_DEVICE_FLAGS_MSI can be changed to any other name if it >>> collides with an already supported vfio-pci or if not necessary, we >>> can remove this flag. >> >> If nothing else you're using a global flag to describe a platform >> device specific augmentation. We've recently added capabilities on the >> device info return that would be more appropriate for this, but >> fundamentally I don't understand why the irq info isn't sufficient. >> Thanks, >> >> Alex >>