Re: [PATCH] virtio: Prevent double swap due to target pre 1.0 VirtIO

[Greg Kurz <groug@kaod.org>]

On Thu, 9 Jan 2020 18:18:57 -0300
André Silva <afscoelho@gmail.com> wrote:

> Hi Greg,
> 

Hi André,

> Thanks for the commit info.
> But I'm testing in this scenario, that is, a ppc64le host with a ppc64
> BE guest, and without my patch I can't get virtio to work. The patch

What are the symptoms without your patch ? What's the QEMU version ?

> makes virtio 0.95 (legacy) net, scsi, blk work. I don't get the
> firmware error. I also tested with a ppc64le guest and had no problems
> either. Maybe we have different firmware versions?
> 
> My firmware output:
> 
> SLOF **********************************************************************
> QEMU Starting
>  Build Date = Jul  3 2019 12:26:14
>  FW Version = git-ba1ab360eebe6338

I'm using the latest SLOF from the QEMU tree (pc-bios/slof.bin):

SLOF **********************************************************************
QEMU Starting
 Build Date = Dec 17 2019 11:31:13
 FW Version = git-9546892a80d5a4c7

Do you hit the issue with upstream QEMU ?

>  Press "s" to enter Open Firmware.
> 
> Populating /vdevice methods
> Populating /vdevice/vty@71000000
> Populating /vdevice/nvram@71000001
> Populating /vdevice/v-scsi@71000002
>        SCSI: Looking for devices
>           8200000000000000 CD-ROM   : "QEMU     QEMU CD-ROM      2.5+"
> Populating /pci@800000020000000
>                      00 0000 (D) : 1af4 1000    virtio [ net ]
>                      00 0800 (D) : 1af4 1001    virtio [ block ]
> No NVRAM common partition, re-initializing...
> Scanning USB
> Using default console: /vdevice/vty@71000000
> 
>   Welcome to Open Firmware
> 
>   Copyright (c) 2004, 2017 IBM Corporation All rights reserved.
>   This program and the accompanying materials are made available
>   under the terms of the BSD License available at
>   http://www.opensource.org/licenses/bsd-license.php
> 
> 
> Trying to load:  from: /pci@800000020000000/scsi@1 ...   Successfully loaded
> 
> >> FreeBSD/powerpc Open Firmware boot block
>    Boot path:   /pci@800000020000000/scsi@1
>    Boot loader: /boot/loader
>    Boot volume:   /pci@800000020000000/scsi@1:2
> Consoles: Open Firmware console
> 
> FreeBSD/powerpc64 Open Firmware loader, Revision 0.1
> (Mon Nov 11 22:33:43 -02 2019 jenkins@FreeBSD_x86)
> Memory: 4194304KB
> Booted from: /pci@800000020000000/scsi@1
> 
> Loading /boot/defaults/loader.conf
> /boot/kernel/kernel data=0x129f658+0x4aaa88 syms=[0x8+0x105120+0x8+0x125429]
> ...
> 
> Until now, I was able to test the patch and see virtio working on the
> following systems:
> 
>   Qemu     Host             Guest                           Guest VirtIO
>   -------- ---------------- ------------------------------- --------------
>   master   Ubuntu ppc64le   FreeBSD 13.0-current ppc64 BE   legacy
>   master   Ubuntu ppc64le   debian 4.19.0-6-powerpc64le     modern
>   master   Ubuntu ppc64le   debian 4.19.0-6-powerpc64le     legacy
>   master   arch x86_64      FreeBSD 13.0-current ppc64 BE   legacy
> 
> Thanks,
> andré
> 
> On Thu, Jan 9, 2020 at 1:06 PM Greg Kurz <groug@kaod.org> wrote:
> >
> > On Thu, 9 Jan 2020 07:39:17 -0500
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >
> > > On Thu, Jan 09, 2020 at 09:25:42AM -0300, André Silva wrote:
> > > > Hi Michael!
> > > > Thanks for reviewing the patch!
> > > >
> > > > > we always get LE values from memory subsystem,
> > > > > not target endian values:
> > > >
> > > > I see. So do you think the patch is correct in eliminating the extra
> > > > swap (as virtio_config_readw for example already makes a swap)?
> > > >
> > > > Thanks,
> > > > andré
> > >
> > > I don't think it is, I think we do need an extra swap
> > > in some cases. It's possible that some cross-endian
> > > setups are broken now, if so pls include testing
> > > result not just theoretical analysis.
> > >
> >
> > I confirm that we must keep the extra swap otherwise
> > read/write in cross-endian setups will have wrong
> > endian. Please read this commit for a more detailed
> > explanation:
> >
> > commit 82afa58641b0e67abbaf4da6c325ebd7c2513262
> > Author: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Date:   Tue Jan 10 01:35:11 2012 +0000
> >
> >     virtio-pci: Fix endianness of virtio config
> >
> > https://git.qemu.org/?p=qemu.git;a=commitdiff;h=82afa58641b0e67abbaf4da6c325ebd7c2513262
> >
> > This is especially critical on ppc64 since _all_ hosts are now LE
> > but the first piece of code in the guest that is likely to drive
> > the device is the SLOF firmware which is BE.
> >
> > This is what we get with this patch when trying to run a pseries guest on a
> > ppc64le host:
> >
> > Trying to load:  from: /pci@800000020000000/scsi@0 ... virtioblk_transfer: Access beyond end of device!
> >
> > Cheers,
> >
> > --
> > Greg
> >
> > > > On Thu, Jan 9, 2020 at 7:50 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > >
> > > > > On Wed, Jan 08, 2020 at 01:16:18PM -0300, Andre Silva wrote:
> > > > > > Remove the bswap function calls after reading and before writing
> > > > > > memory bytes in virtio_pci_config_read and virtio_pci_config_write
> > > > > > because they are reverting back an already swapped bytes.
> > > > > >
> > > > > > Consider the table below in the context of virtio_pci_config_read
> > > > > > function.
> > > > > >
> > > > > > Host   Target  virtio-config-read[wl]
> > > > > >                swap?                   virtio-is-big-endian?   extra bswap?   Should be   Final result   Final result ok?
> > > > > > ----- ------- ------------------------ ----------------------- -------------- ----------- -------------- ------------------
> > > > > > LE     BE      s(x)                    true                    s(s(x))        s(x)        x              No
> > > > > > LE     LE      x                       false                   -              x           x              Yes
> > > > > > BE     LE      s(x)                    false                   -              s(x)        s(x)           Yes
> > > > > > BE     BE      x                       true                    s(x)           x           s(x)           No
> > > > >
> > > > > we always get LE values from memory subsystem,
> > > > > not target endian values:
> > > > >
> > > > > static const MemoryRegionOps virtio_pci_config_ops = {
> > > > >     .read = virtio_pci_config_read,
> > > > >     .write = virtio_pci_config_write,
> > > > >     .impl = {
> > > > >         .min_access_size = 1,
> > > > >         .max_access_size = 4,
> > > > >     },
> > > > >     .endianness = DEVICE_LITTLE_ENDIAN,
> > > > > };
> > > > >
> > > > >
> > > > > This triggers another swap in address_space_ldl_internal
> > > > > (memory_ldst.inc.c).
> > > > >
> > > > >
> > > > > > In table above, when target is big endian and VirtIO is pre 1.0,
> > > > > > function virtio_is_big_endian would return true and the extra
> > > > > > swap would be executed, reverting the previous swap made by
> > > > > > virtio_config_read[wl].
> > > > > >
> > > > > > The 's(x)' means that a swap function was applied at
> > > > > > address x. 'LE' is little endian and 'BE' is big endian. The
> > > > > > 'Final result' column is the returned value from
> > > > > > virtio_pci_config_read, considering a target Virtio pre 1.0.
> > > > > > 'x' means that target's value was not swapped in Qemu, 's(x)' means
> > > > > > that Qemu will use a swapped value.
> > > > > >
> > > > > > If we remove the extra swap made in virtio_pci_config_read we will
> > > > > > have the correct result in any host/target combination, both for
> > > > > > VirtIO pre 1.0 or later versions.
> > > > > >
> > > > > > The same reasoning applies to virtio_pci_config_write.
> > > > > >
> > > > > > Signed-off-by: Andre Silva <afscoelho@gmail.com>
> > > > > > ---
> > > > > >  hw/virtio/virtio-pci.c | 12 ------------
> > > > > >  1 file changed, 12 deletions(-)
> > > > > >
> > > > > > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > > > > > index c6b47a9c73..4ba9e847f3 100644
> > > > > > --- a/hw/virtio/virtio-pci.c
> > > > > > +++ b/hw/virtio/virtio-pci.c
> > > > > > @@ -431,15 +431,9 @@ static uint64_t virtio_pci_config_read(void *opaque, hwaddr addr,
> > > > > >          break;
> > > > > >      case 2:
> > > > > >          val = virtio_config_readw(vdev, addr);
> > > > > > -        if (virtio_is_big_endian(vdev)) {
> > > > > > -            val = bswap16(val);
> > > > > > -        }
> > > > > >          break;
> > > > > >      case 4:
> > > > > >          val = virtio_config_readl(vdev, addr);
> > > > > > -        if (virtio_is_big_endian(vdev)) {
> > > > > > -            val = bswap32(val);
> > > > > > -        }
> > > > > >          break;
> > > > > >      }
> > > > > >      return val;
> > > > > > @@ -465,15 +459,9 @@ static void virtio_pci_config_write(void *opaque, hwaddr addr,
> > > > > >          virtio_config_writeb(vdev, addr, val);
> > > > > >          break;
> > > > > >      case 2:
> > > > > > -        if (virtio_is_big_endian(vdev)) {
> > > > > > -            val = bswap16(val);
> > > > > > -        }
> > > > > >          virtio_config_writew(vdev, addr, val);
> > > > > >          break;
> > > > > >      case 4:
> > > > > > -        if (virtio_is_big_endian(vdev)) {
> > > > > > -            val = bswap32(val);
> > > > > > -        }
> > > > > >          virtio_config_writel(vdev, addr, val);
> > > > > >          break;
> > > > > >      }
> > > > > > --
> > > > > > 2.24.1
> > > > >
> > >
> > >
> >