Re: intel_iommu unit test is also failing

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 05, 2026 at 01:38:26PM +0200, Thomas Huth wrote:
> On 05/05/2026 12.53, Michael S. Tsirkin wrote:
> > On Tue, May 05, 2026 at 12:34:41PM +0200, Thomas Huth wrote:
> > > On 05/05/2026 12.23, Michael S. Tsirkin wrote:
> > > > On Tue, May 05, 2026 at 11:45:17AM +0200, Thomas Huth wrote:
> > > > > On 05/05/2026 11.27, Clément MATHIEU--DRIF wrote:
> > > > > > I had a bit more time to hook into qemu to check the root cause.
> > > > > > 
> > > > > > It seems that testb issues a single byte read (out of the valid size range), as we can see on the following breakpoint:
> > > > > > 
> > > > > > ```
> > > > > > Thread 6 "CPU 0/TCG" hit Breakpoint 2, memory_region_dispatch_read (mr=0x55d72883cb30, addr=152, pval=0x7f62d25f4590, op=MO_BSWAP, attrs=...) at ../system/memory.c:1473
> > > > > > 1473       unsigned size = memop_size(op);
> > > > > > (gdb) n
> > > > > > 1474       MemTxResult r;
> > > > > > (gdb) p size
> > > > > > $1 = 1
> > > > > > (gdb)
> > > > > > ```
> > > > > 
> > > > > Ouch! That's an excellent finding, Clément ... so GCC 16 is "smart" enough
> > > > > to see that we only want to test the lowest bit here, so it optimizes the
> > > > > code to access only one byte of memory instead of 4 bytes... which would be
> > > > > ok for normal memory, but not for an MMIO register :-/
> > > > > 
> > > > > Ugly work-around, to force GCC to read 32 bits:
> > > > > 
> > > > > diff --git a/lib/asm-generic/io.h b/lib/asm-generic/io.h
> > > > > --- a/lib/asm-generic/io.h
> > > > > +++ b/lib/asm-generic/io.h
> > > > > @@ -38,7 +38,9 @@ static inline u16 __raw_readw(const volatile void *addr)
> > > > >    #ifndef __raw_readl
> > > > >    static inline u32 __raw_readl(const volatile void *addr)
> > > > >    {
> > > > > -       return *(const volatile u32 *)addr;
> > > > > +       u32 val = *(const volatile u32 *)addr;
> > > > > +       asm volatile ("\n" : : "r"(addr));
> > > > > +       return val;
> > > > >    }
> > > > >    #endif
> > > > > 
> > > > > ... but I wonder whether this should rather be treated as a bug in GCC
> > > > > instead, since it should IMHO really not change the access size for a
> > > > > volatile memory access?
> > > > > 
> > > > >    Thomas
> > > > 
> > > > Wouldn't this break linux generally?
> > > > 
> > > > #ifndef __READ_ONCE
> > > > #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
> > > > #endif
> > > 
> > > I asked myself the very same question, but after googling for "GCC 16 linux
> > > kernel" issues, I did not find anything related... there is likely something
> > > specific to kvm-unit-tests in here...
> > > 
> > >   Thomas
> > 
> > 
> > This seems to be pertinent:
> > https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
> > 
> > -ffuse-ops-with-volatile-access
> > Allow limited optimization of operations with volatile memory access when doing so does not change the semantics outlined in See When is a Volatile Object Accessed?.
> > 
> > The default is -ffuse-ops-with-volatile-access
> > 
> > implemented here:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122343
> > 
> > Try disabling? -fno-fuse-ops-with-volatile-access
> 
> Thanks, this seems to fix the issue, indeed!
> 
> Would you like to send a patch for it?
> 
>  Thomas


So then it's this bug apparently:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125180

will likely be in 16.2?

Maybe just wait?


-- 
MST