Re: intel_iommu unit test is also failing (was: Re: [PATCH kvm-unit-tests] realmode: load above stack)

["Clément MATHIEU--DRIF" <clement.mathieu--drif@bull.com>]

Back with some answers:

This is the incriminated hunk:

```diff
--- <unnamed>
+++ <unnamed>
@@ -1,17 +1,16 @@
-  404395:       8b 80 98 00 00 00       mov    0x98(%eax),%eax
+  40441d:       8b 43 38                mov    0x38(%ebx),%eax
         edu_reg_writeq(dev, EDU_REG_DMA_DST, to);
         edu_reg_writeq(dev, EDU_REG_DMA_COUNT, size);
         edu_reg_writel(dev, EDU_REG_DMA_CMD, cmd);

         /* Wait until DMA finished */
         while (edu_reg_readl(dev, EDU_REG_DMA_CMD) & EDU_CMD_DMA_START)
-  40439b:       a8 01                   test   $0x1,%al
-  40439d:       74 10                   je     4043af <edu_dma+0x121>
-  40439f:       f3 90                   pause
-  4043a1:       48                      dec    %eax
-  4043a2:       8b 43 38                mov    0x38(%ebx),%eax
-  4043a5:       8b 80 98 00 00 00       mov    0x98(%eax),%eax
-  4043ab:       a8 01                   test   $0x1,%al
-  4043ad:       75 f0                   jne    40439f <edu_dma+0x111>
+  404420:       f6 80 98 00 00 00 01    testb  $0x1,0x98(%eax)
+  404427:       74 0f                   je     404438 <edu_dma+0x11f>
+  404429:       f3 90                   pause
+  40442b:       48                      dec    %eax
+  40442c:       8b 43 38                mov    0x38(%ebx),%eax
+  40442f:       f6 80 98 00 00 00 01    testb  $0x1,0x98(%eax)
+  404436:       75 f1                   jne    404429 <edu_dma+0x110>
                 cpu_relax();
 }

+ is gcc 16
- is gcc 15

The instructions generated by gcc 16 always skip the following condition:

```
        /* Wait until DMA finished */
        while (edu_reg_readl(dev, EDU_REG_DMA_CMD) & EDU_CMD_DMA_START)
                cpu_relax();
```

As a consequence, the test performs the second dma operation too early and reads a wrong value.

Regards,
cmd

On Tue, 2026-05-05 at 08:37 +0200, Clement Mathieu--Drif wrote:
> I will try to investigate today, it seems that the host does not wait for the dma operation to complete before reading back.
>
> keep you posted
>
> cmd
>
> On Tue, 2026-05-05 at 07:49 +0200, Clement Mathieu--Drif wrote:
>
> > Hi,
> >
> > Indeed, it seems to start failing when switching to gcc 16.1.
> >
> > gcc 15.2.1 - Qemu 11 => pass
> > gcc 16.1 - Qemu 11 => fail
> >
> > On Mon, 2026-05-04 at 11:45 -0400, Peter Xu wrote:
> >
> >
> > > Caution: External email. Do not open attachments or click links, unless this email comes from a known sender and you know the content is safe.
> > >
> > >
> > > On Mon, May 04, 2026 at 10:07:25AM +0200, Thomas Huth wrote:
> > >
> > >
> > >
> > > > On 04/05/2026 09.58, Thomas Huth wrote:
> > > >
> > > >
> > > >
> > > > > On 04/06/2024 16.35, Paolo Bonzini wrote:
> > > > >
> > > > >
> > > > >
> > > > > > The bottom 32K of memory are generally reserved for use by the BIOS;
> > > > > > for example, traditionally the boot loader is placed at 0x7C00 and
> > > > > > the stack grows below that address.
> > > > > >
> > > > > > It turns out that with some versions of clang, realmode.flat has
> > > > > > become big enough that it overlaps the stack used by the multiboot
> > > > > > option ROM loader.  The result is that a couple instructions are
> > > > > > overwritten.  Typically one or two tests fail and that's it...
> > > > > >
> > > > > > Move the code above the forbidden region, in real 90s style.
> > > > > >
> > > > > > Reported-by: Thomas Huth <[[[thuth@redhat.com](mailto:thuth@redhat.com)](mailto:[thuth@redhat.com](mailto:thuth@redhat.com))](mailto:[[thuth@redhat.com](mailto:thuth@redhat.com)](mailto:[thuth@redhat.com](mailto:thuth@redhat.com)))>
> > > > > > Signed-off-by: Paolo Bonzini <[[[pbonzini@redhat.com](mailto:pbonzini@redhat.com)](mailto:[pbonzini@redhat.com](mailto:pbonzini@redhat.com))](mailto:[[pbonzini@redhat.com](mailto:pbonzini@redhat.com)](mailto:[pbonzini@redhat.com](mailto:pbonzini@redhat.com)))>
> > > > > > ---
> > > > > >   x86/realmode.lds | 2 +-
> > > > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/x86/realmode.lds b/x86/realmode.lds
> > > > > > index 0ed3063b..e4782a98 100644
> > > > > > --- a/x86/realmode.lds
> > > > > > +++ b/x86/realmode.lds
> > > > > > @@ -1,6 +1,6 @@
> > > > > >   SECTIONS
> > > > > >   {
> > > > > > -    . = 16K;
> > > > > > +    . = 32K;
> > > > > >       stext = .;
> > > > > >       .text : { *(.init) *(.text) }
> > > > > >       . = ALIGN(4K);
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >   Hi Paolo!
> > > > >
> > > > > FYI, the realmode kvm-unit-test now also fails with the recent version
> > > > > of GCC 16 for the i386 target:
> > > > >
> > > > >   [[[https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727))](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727)))
> > > > >
> > > > > It was working fine some weeks ago with GCC 15.1:
> > > > >
> > > > >   [[[https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961))](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961)))
> > > > >
> > > > > When I apply your patch, the problem seems to be gone again in this
> > > > > case, but since there were some other issues with this (with older
> > > > > versions of GCC, I think):
> > > > >
> > > > > [[[https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/](https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/)](https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/](https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/))](https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/](https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/)](https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/](https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/)))
> > > > >
> > > > > ... there must be a better way to fix it?
> > > > >
> > > > > Could you please have a look?
> > > >
> > > >
> > > >
> > > >
> > > > By the way, the intel_iommu test now also suddenly started failing (for the
> > > > x86_64 target), either due to update of GCC or due to the update from QEMU
> > > > v10.2 to 11.0 :
> > > >
> > > >  [[[https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728))](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195728)))
> > > >
> > > > Two weeks ago, it was still working fine:
> > > >
> > > >  [[[https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962))](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962)](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962](https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260962)))
> > >
> > >
> > >
> > >
> > > Looping in those who take care of qemu's VT-D now (Yi, Clément)..
> > >
> > > --
> > > Peter Xu
> > >
> >
> >
>