Re: [PULL 1/2] amd_iommu: Fix pte_override_page_mask()

[Jean-Philippe Brucker]

On Fri, Apr 23, 2021 at 02:01:19PM +0100, Peter Maydell wrote:
> On Thu, 22 Apr 2021 at 23:24, Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > From: Jean-Philippe Brucker <jean-philippe@linaro.org>
> >
> > AMD IOMMU PTEs have a special mode allowing to specify an arbitrary page
> > size. Quoting the AMD IOMMU specification: "When the Next Level bits [of
> > a pte] are 7h, the size of the page is determined by the first zero bit
> > in the page address, starting from bit 12."
> >
> > So if the lowest bits of the page address is 0, the page is 8kB. If the
> > lowest bits are 011, the page is 32kB. Currently pte_override_page_mask()
> > doesn't compute the right value for this page size and amdvi_translate()
> > can return the wrong guest-physical address. With a Linux guest, DMA
> > from SATA devices accesses the wrong memory and causes probe failure:
> >
> > qemu-system-x86_64 ... -device amd-iommu -drive id=hd1,file=foo.bin,if=none 
> > \
> >                 -device ahci,id=ahci -device ide-hd,drive=hd1,bus=ahci.0
> > [    6.613093] ata1.00: qc timeout (cmd 0xec)
> > [    6.615062] ata1.00: failed to IDENTIFY (I/O error, err_mask=0x4)
> >
> > Fix the page mask.
> >
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > Message-Id: <20210421084007.1190546-1-jean-philippe@linaro.org>
> > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> Jean-Philippe, do you know if this is a regression since 5.2?

I don't think so, I can reproduce it with v5.2.0.

> I'm guessing not given that the function in question has been that
> way since the amd_iommu was introduced in 2016.

There has been a lot of work on the AMD IOMMU driver in Linux recently.
Maybe that exacerbated the problem but I can't find a relevant change.
It's also possible that this path hasn't been exercised before - I just
happened to run a SATA device under AMD IOMMU this week to debug an
unrelated Linux issue. The other devices in the VM don't seem to have a
problem doing DMA.

Thanks,
Jean