From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Gibson Subject: Re: [RFC PATCH kernel] vfio-pci: Allow mapping MSIX BAR Date: Wed, 22 Nov 2017 15:44:55 +1100 Message-ID: <20171122044455.GR2380@umbus.fritz.box> References: <20171122040932.970-1-aik@ozlabs.ru> <20171121212846.4d567f2e@t450s.home> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="Hw0FrjWlp+qkNlJP" Cc: Alexey Kardashevskiy , kvm@vger.kernel.org, Yongji Xie , Eric Auger To: Alex Williamson Return-path: Received: from ozlabs.org ([103.22.144.67]:39479 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751772AbdKVEo7 (ORCPT ); Tue, 21 Nov 2017 23:44:59 -0500 Content-Disposition: inline In-Reply-To: <20171121212846.4d567f2e@t450s.home> Sender: kvm-owner@vger.kernel.org List-ID: --Hw0FrjWlp+qkNlJP Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Nov 21, 2017 at 09:28:46PM -0700, Alex Williamson wrote: > On Wed, 22 Nov 2017 15:09:32 +1100 > Alexey Kardashevskiy wrote: >=20 > > By default VFIO disables mapping of MSIX BAR to the userspace as > > the userspace may program it in a way allowing spurious interrupts; > > instead the userspace uses the VFIO_DEVICE_SET_IRQS ioctl. > >=20 > > This works fine as long as the system page size equals to the MSIX > > alignment requirement which is 4KB. However with a bigger page size > > the existing code prohibits mapping non-MSIX parts of a page with MSIX > > structures so these parts have to be emulated via slow reads/writes on > > a VFIO device fd. If these emulated bits are accessed often, this has > > serious impact on performance. > >=20 > > This adds an ioctl to the vfio-pci device which hides the sparse > > capability and allows the userspace to map a BAR with MSIX structures. >=20 > So the user is in control of telling the kernel whether they're allowed > to mmap the msi-x vector table. That makes absolutely no sense. If > you're trying to figure out how userspace knows whether to implicitly > avoid mmap'ing the msix region, I think there are far better ways in > the existing region info ioctl. We could use a flag, or maybe the > existence of a capability chain pointer, or a new capability. But > absolutely not this. The kernel needs to decide whether it's going to > let the user do this, not the user. Thanks, No, it doesn't. This is actually the approach we discussed in Prague. Remember that intercepting access to the MSI-X table is not a host safety / security issue. It's just that without that we won't wire up the device's MSI-X vectors properly so they won't work. Basically the decision here is between A) Allow MSI-X configuration via standard PCI mechanisms, at the cost of making access slow for any registers sharing a page with the MSI-X table. or B) Make access to BAR registers sharing a page with the MSI-X table fast, at the cost of requiring some alternative mechanism to configure MSI-X vectors. And that is a tradeoff that it is reasonable for userspace to make. In the case of KVM guests, the decision depends entirely on the *guest* platform. Usually we need (A) because the guest expects to be able to poke the MSI-X table in the usual way. However for PAPR guests, there's an alternative mechanism via an RTAS call, which means we can use (B). The host kernel can't make this decision, because it doesn't know the guest platform (well, KVM might, but VFIO doesn't). A userspace VFIO program could also elect for (B) if it does care about performance of access to registers in the same BAR as the MSI-X table, but doesn't need MSI-X for example. --=20 David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson --Hw0FrjWlp+qkNlJP Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEdfRlhq5hpmzETofcbDjKyiDZs5IFAloVAMQACgkQbDjKyiDZ s5LJaRAAonk3SzzhmzwIAXU0E3OxiyusnQDQXrB5rfcUvPC4mRcQCtg5fjuOQL+l upc+x+6McioieTP3vq3Gdr1QyoxQnMhV13E/MHeL7BbfSSChwKb4s2c4bOUHcN+2 7fsXNAOL6hqdpuCDuwcYbFd48MBYsifQzt18oVLnz9L94+YbK2yFvlfEJALF8sZk 7H+FZKc79zeeMiVRFW0bU529nGlJ8UlVqGNFaP0SYbfloSmCk6jamibvTIp2ZW1H P53XMjfr+p7KnjtpkxrAlQTV2uXS/FLxR6U3V20EIdXVCDCMcPXIMmDH5x/JAP5k WM6HBgtS79w5WXlyhHXZoQzsJMd2cjZDuAxgadNEaaWQ8T6ukC2OzeDNy0/XhPAZ 4YsU3UjBLvu+yod/w4gc+5uWQh7e8WmfAjVgPS42BaFl357W4o50c0/I/jaZHlhX z4XZGQeZQgDSGWm/QGXkgVPrc3JnktHzfqWZ4jvongxNva0Sth8GIrCrCNm1MhSq 9xqiTNhoda59TSUR/XgRpexLhDxF3DmL9PD6vq4DwSmcpr4a5dCV4vKsEZRV+0kv Dok0IBnAABz2rrBQA00Ah1KUkhAHuKp4G7J8uv8QKCUmFIV1QjtpFHdBNKdmQE23 MLw3QRExaehlnmfYUoMX0gd+lfrxfWu7/yoJH0yxECwJ5MIZO5E= =RcvG -----END PGP SIGNATURE----- --Hw0FrjWlp+qkNlJP--