Re: Virtio_pci in kernel ignore endian of PCI I/O space?

Re: Virtio_pci in kernel ignore endian of PCI I/O space?

[Hollis Blanchard]

On Tue, 2008-09-02 at 15:29 +0800, Liu Yu-B13201 wrote:
> Hollis,
> 
> I noticed fuction virtio_blk_update_config() (qemu/hw/virtio-blk.c) in
> qemu
> updated the virtio disk's capacity in little endian.
> But virtblk_probe() (drivers/block/virtio_blk.c) in kernel read the
> capacity from I/O space without convertion.

Sigh, I don't remember why this works any more.

There was discussion on this on kvm-ppc-devel in April (unfortunately it
was right at the month line, so the threading is broken, but see e.g.
http://marc.info/?l=kvm-ppc&m=120716463602156), and ultimately
cpu_to_le64() was added to qemu. However, a couple weeks later, the
swapping that the kernel was doing was removed (see
http://thread.gmane.org/gmane.linux.kernel.virtualization/5776/focus=5801).

However, the code is actually working today on 440 (and reporting a sane
number of blocks). Also, this patch
(http://article.gmane.org/gmane.linux.kernel.virtualization/6126/match=virtio%5fblk+fix+endianess+annotations) suggests that struct virtio_blk_config is no longer LE, so I'm thinking your problem must be in qemu. AFAICS the kernel is basically just doing memcpy in a few places.

> Since E500 is big endian, this bring the misunderstanding between qemu
> and guest.

What are you using for PCI emulation in qemu? I don't think it should
matter in this case, since the kernel is doing 1-byte reads in vp_get(),
but endianness gets very convoluted wherever qemu is involved. :(

-- 
Hollis Blanchard
IBM Linux Technology Center

RE: Virtio_pci in kernel ignore endian of PCI I/O space?

[Liu Yu-B13201]

> -----Original Message-----
> From: Hollis Blanchard [mailto:hollisb-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org] 
> Sent: Tuesday, September 02, 2008 10:16 PM
> To: Liu Yu-B13201
> Cc: kvm-ppc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; kvm
> Subject: Re: Virtio_pci in kernel ignore endian of PCI I/O space?
> 
> On Tue, 2008-09-02 at 15:29 +0800, Liu Yu-B13201 wrote:
> > Hollis,
> > 
> > I noticed fuction virtio_blk_update_config() 
> (qemu/hw/virtio-blk.c) in
> > qemu
> > updated the virtio disk's capacity in little endian.
> > But virtblk_probe() (drivers/block/virtio_blk.c) in kernel read the
> > capacity from I/O space without convertion.
> 
> Sigh, I don't remember why this works any more.
> 
> There was discussion on this on kvm-ppc-devel in April 
> (unfortunately it
> was right at the month line, so the threading is broken, but see e.g.
> http://marc.info/?l=kvm-ppc&m=120716463602156), and ultimately
> cpu_to_le64() was added to qemu. However, a couple weeks later, the
> swapping that the kernel was doing was removed (see
> http://thread.gmane.org/gmane.linux.kernel.virtualization/5776
> /focus=5801).
> 
> However, the code is actually working today on 440 (and 
> reporting a sane
> number of blocks). Also, this patch
> (http://article.gmane.org/gmane.linux.kernel.virtualization/61
> 26/match=virtio%5fblk+fix+endianess+annotations) suggests 
> that struct virtio_blk_config is no longer LE, so I'm 
> thinking your problem must be in qemu. AFAICS the kernel is 
> basically just doing memcpy in a few places.

Thanks for your informations!

> 
> > Since E500 is big endian, this bring the misunderstanding 
> between qemu
> > and guest.
> 
> What are you using for PCI emulation in qemu? I don't think it should
> matter in this case, since the kernel is doing 1-byte reads 
> in vp_get(),
> but endianness gets very convoluted wherever qemu is involved. :(
> 

Did you see Anthony's reply?
It's weird that 440 can still work fine.

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RE: Virtio_pci in kernel ignore endian of PCI I/O space?

[Hollis Blanchard]

On Thu, 2008-09-04 at 13:27 +0800, Liu Yu-B13201 wrote:
> > 
> > > Since E500 is big endian, this bring the misunderstanding 
> > between qemu
> > > and guest.
> > 
> > What are you using for PCI emulation in qemu? I don't think it should
> > matter in this case, since the kernel is doing 1-byte reads 
> > in vp_get(),
> > but endianness gets very convoluted wherever qemu is involved. :(
> > 
> 
> Did you see Anthony's reply?
> It's weird that 440 can still work fine.

OK, I've found my problem. My host kernel is current, but my *guest* 
kernel is old (it was still byte-swapping).

Also, the reason the S390 guys didn't see a problem is that they aren't
yet using qemu, and their virtio userspace implementation doesn't swap.

Yu, would you send a patch?

-- 
Hollis Blanchard
IBM Linux Technology Center

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RE: Virtio_pci in kernel ignore endian of PCI I/O space?

[Liu Yu-B13201]

> -----Original Message-----
> From: Hollis Blanchard [mailto:hollisb-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org] 
> Sent: Friday, September 05, 2008 11:31 PM
> To: Liu Yu-B13201
> Cc: kvm-ppc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org; kvm
> Subject: RE: Virtio_pci in kernel ignore endian of PCI I/O space?
> 
> On Thu, 2008-09-04 at 13:27 +0800, Liu Yu-B13201 wrote:
> > > 
> > > > Since E500 is big endian, this bring the misunderstanding 
> > > between qemu
> > > > and guest.
> > > 
> > > What are you using for PCI emulation in qemu? I don't 
> think it should
> > > matter in this case, since the kernel is doing 1-byte reads 
> > > in vp_get(),
> > > but endianness gets very convoluted wherever qemu is involved. :(
> > > 
> > 
> > Did you see Anthony's reply?
> > It's weird that 440 can still work fine.
> 
> OK, I've found my problem. My host kernel is current, but my *guest* 
> kernel is old (it was still byte-swapping).
> 
> Also, the reason the S390 guys didn't see a problem is that 
> they aren't
> yet using qemu, and their virtio userspace implementation 
> doesn't swap.

Thanks a lot!

> 
> Yu, would you send a patch?
> 

Certainly.
As my code is on branch kvm-70rc1, I could only test it on branch
kvm-70rc1 now.
I don't know whether it is matter?

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