From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Woodhouse Subject: Re: [PATCH v5 04/10] vring: Introduce vring_use_dma_api() Date: Mon, 01 Feb 2016 11:22:03 +0000 Message-ID: <1454325723.133285.285.camel__11355.3057112314$1454325825$gmane$org@infradead.org> References: Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============1243786172378095306==" Return-path: Received: from mail6.bemta4.messagelabs.com ([85.158.143.247]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1aQCYu-0001Y2-4v for xen-devel@lists.xenproject.org; Mon, 01 Feb 2016 11:22:28 +0000 In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andy Lutomirski , "Michael S. Tsirkin" Cc: Cornelia Huck , Joerg Roedel , KVM , linux-s390 , Benjamin Herrenschmidt , Stefano Stabellini , Sebastian Ott , "linux-kernel@vger.kernel.org" , Christoph Hellwig , Christian Borntraeger , David Vrabel , xen-devel@lists.xenproject.org, sparclinux@vger.kernel.org, Paolo Bonzini , Linux Virtualization , "David S. Miller" , Martin Schwidefsky List-Id: xen-devel@lists.xenproject.org --===============1243786172378095306== Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-b1Gw/uWIDRqexPnzAjCF" --=-b1Gw/uWIDRqexPnzAjCF Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2016-01-28 at 18:31 -0800, Andy Lutomirski wrote: > This is a kludge, but no one has come up with a a better idea yet. > We'll introduce DMA API support guarded by vring_use_dma_api(). > Eventually we may be able to return true on more and more systems, > and hopefully we can get rid of vring_use_dma_api() entirely some > day. >=20 > Signed-off-by: Andy Lutomirski > --- > =C2=A0drivers/virtio/virtio_ring.c | 24 ++++++++++++++++++++++++ > =C2=A01 file changed, 24 insertions(+) >=20 > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > index e12e385f7ac3..4b8dab4960bb 100644 > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -25,6 +25,30 @@ > =C2=A0#include=20 > =C2=A0#include=20 > =C2=A0 > +/* > + * The interaction between virtio and a possible IOMMU is a mess. > + * > + * On most systems with virtio, physical addresses match bus addresses, > + * and it doesn't particularly matter whether we use the DMI API. > + * > + * On some sytems, including Xen and any system with a physical device > + * that speaks virtio behind a physical IOMMU, we must use the DMA API > + * for virtio DMA to work at all. > + * > + * On other systems, including SPARC and PPC64, virtio-pci devices are > + * enumerated as though they are behind an IOMMU, but the virtio host > + * ignores the IOMMU, so we must either pretend that the IOMMU isn't > + * there or somehow map everything as the identity. > + * > + * For the time being, we preseve historic behavior and bypass the DMA > + * API. > + */ I spot at least three typos in there, FWIW. ('DMI API', 'sytems', 'preseve'). > +static bool vring_use_dma_api(void) > +{ > + return false; > +} > + I'd quite like to see this be an explicit opt-out for the known-broken platforms. We've listed the SPARC and PPC64 issues. For x86 I need to refresh my memory as a prelude to trying to fix it... was the issue *just* that Qemu tends to ship with a broken BIOS that misdescribes the virtio devices (and any assigned PCI devices) as being behind an IOMMU when they're not, in the rare case that Qemu actually exposes its partially-implemented virtual IOMMU to the guest? Could we have an arch_vring_eschew_dma_api(dev) function which the affected architectures could provide (as a prelude to fixing it so that the DMA API does the right thing for *itself*)? It would be functionally equivalent, but it would help to push the workarounds to the right place =E2=80=94 rather than entrenching them for e= ver in tricky "OMG we need to audit what all the architectures do... let's not touch it!" code. --=20 David Woodhouse Open Source Technology Centre David.Woodhouse@intel.com Intel Corporation --=-b1Gw/uWIDRqexPnzAjCF Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIISjjCCBicw ggUPoAMCAQICAw3vNzANBgkqhkiG9w0BAQUFADCBjDELMAkGA1UEBhMCSUwxFjAUBgNVBAoTDVN0 YXJ0Q29tIEx0ZC4xKzApBgNVBAsTIlNlY3VyZSBEaWdpdGFsIENlcnRpZmljYXRlIFNpZ25pbmcx ODA2BgNVBAMTL1N0YXJ0Q29tIENsYXNzIDEgUHJpbWFyeSBJbnRlcm1lZGlhdGUgQ2xpZW50IENB MB4XDTE1MDUwNTA5NDM0MVoXDTE2MDUwNTA5NTMzNlowQjEcMBoGA1UEAwwTZHdtdzJAaW5mcmFk ZWFkLm9yZzEiMCAGCSqGSIb3DQEJARYTZHdtdzJAaW5mcmFkZWFkLm9yZzCCASIwDQYJKoZIhvcN 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