Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752808AbaFSAvF (ORCPT ); Wed, 18 Jun 2014 20:51:05 -0400 Received: from mail-pd0-f179.google.com ([209.85.192.179]:54375 "EHLO mail-pd0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751632AbaFSAvD (ORCPT ); Wed, 18 Jun 2014 20:51:03 -0400 Message-ID: <53A233E9.6030006@ozlabs.ru> Date: Thu, 19 Jun 2014 10:50:49 +1000 From: Alexey Kardashevskiy User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Alex Williamson CC: linuxppc-dev@lists.ozlabs.org, Benjamin Herrenschmidt , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Graf , Nikunj A Dadhania Subject: Re: [PATCH] vfio: Fix endianness handling for emulated BARs References: <1403091391-31780-1-git-send-email-aik@ozlabs.ru> <1403116512.3707.175.camel@ul30vt.home> In-Reply-To: <1403116512.3707.175.camel@ul30vt.home> Content-Type: text/plain; charset=KOI8-R Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/19/2014 04:35 AM, Alex Williamson wrote: > On Wed, 2014-06-18 at 21:36 +1000, Alexey Kardashevskiy wrote: >> VFIO exposes BARs to user space as a byte stream so userspace can >> read it using pread()/pwrite(). Since this is a byte stream, VFIO should >> not do byte swapping and simply return values as it gets them from >> PCI device. >> >> Instead, the existing code assumes that byte stream in read/write is >> little-endian and it fixes endianness for values which it passes to >> ioreadXX/iowriteXX helpers. This works for little-endian as PCI is >> little endian and le32_to_cpu/... are stubs. > > vfio read32: > > val = cpu_to_le32(ioread32(io + off)); > > Where the typical x86 case, ioread32 is: > > #define ioread32(addr) readl(addr) > > and readl is: > > __le32_to_cpu(__raw_readl(addr)); > > So we do canceling byte swaps, which are both nops on x86, and end up > returning device endian, which we assume is little endian. > > vfio write32 is similar: > > iowrite32(le32_to_cpu(val), io + off); > > The implicit cpu_to_le32 of iowrite32() and our explicit swap cancel > out, so input data is device endian, which is assumed little. > >> This also works for big endian but rather by an accident: it reads 4 bytes >> from the stream (@val is big endian), converts to CPU format (which should >> be big endian) as it was little endian (@val becomes actually little >> endian) and calls iowrite32() which does not do swapping on big endian >> system. > > Really? > > In arch/powerpc/kernel/iomap.c iowrite32() is just a wrapper around > writel(), which seems to use the generic implementation, which does > include a cpu_to_le32. Ouch, wrong comment. iowrite32() does swapping. My bad. > > I also see other big endian archs like parisc doing cpu_to_le32 on > iowrite32, so I don't think this statement is true. I imagine it's > probably working for you because the swap cancel. > >> This removes byte swapping and makes use ioread32be/iowrite32be >> (and 16bit versions) on big-endian systems. The "be" helpers take >> native endian values and do swapping at the moment of writing to a PCI >> register using one of "store byte-reversed" instructions. > > So now you want iowrite32() on little endian and iowrite32be() on big > endian, the former does a cpu_to_le32 (which is a nop on little endian) > and the latter does a cpu_to_be32 (which is a nop on big endian)... > should we just be using __raw_writel() on both? We can do that too. The beauty of iowrite32be on ppc64 is that it does not swap and write separately, it is implemented via the "Store Word Byte-Reverse Indexed X-form" single instruction. And some archs (do not know which ones) may add memory barriers in their implementations of ioread/iowrite. __raw_writel is too raw :) > There doesn't actually > seem to be any change in behavior here, it just eliminates back-to-back > byte swaps, which are a nop on x86, but not power, right? Exactly. No dependency for QEMU. > >> Suggested-by: Benjamin Herrenschmidt >> Signed-off-by: Alexey Kardashevskiy >> --- >> drivers/vfio/pci/vfio_pci_rdwr.c | 20 ++++++++++++++++---- >> 1 file changed, 16 insertions(+), 4 deletions(-) >> >> diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c >> index 210db24..f363b5a 100644 >> --- a/drivers/vfio/pci/vfio_pci_rdwr.c >> +++ b/drivers/vfio/pci/vfio_pci_rdwr.c >> @@ -21,6 +21,18 @@ >> >> #include "vfio_pci_private.h" >> >> +#ifdef __BIG_ENDIAN__ >> +#define ioread16_native ioread16be >> +#define ioread32_native ioread32be >> +#define iowrite16_native iowrite16be >> +#define iowrite32_native iowrite32be >> +#else >> +#define ioread16_native ioread16 >> +#define ioread32_native ioread32 >> +#define iowrite16_native iowrite16 >> +#define iowrite32_native iowrite32 >> +#endif >> + >> /* >> * Read or write from an __iomem region (MMIO or I/O port) with an excluded >> * range which is inaccessible. The excluded range drops writes and fills >> @@ -50,9 +62,9 @@ static ssize_t do_io_rw(void __iomem *io, char __user *buf, >> if (copy_from_user(&val, buf, 4)) >> return -EFAULT; >> >> - iowrite32(le32_to_cpu(val), io + off); >> + iowrite32_native(val, io + off); >> } else { >> - val = cpu_to_le32(ioread32(io + off)); >> + val = ioread32_native(io + off); >> >> if (copy_to_user(buf, &val, 4)) >> return -EFAULT; >> @@ -66,9 +78,9 @@ static ssize_t do_io_rw(void __iomem *io, char __user *buf, >> if (copy_from_user(&val, buf, 2)) >> return -EFAULT; >> >> - iowrite16(le16_to_cpu(val), io + off); >> + iowrite16_native(val, io + off); >> } else { >> - val = cpu_to_le16(ioread16(io + off)); >> + val = ioread16_native(io + off); >> >> if (copy_to_user(buf, &val, 2)) >> return -EFAULT; > > > -- Alexey -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/