Re: intel_iommu unit test is also failing

[Thomas Huth <thuth@redhat.com>]

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