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Tue, 05 May 2026 10:08:09 -0700 (PDT) Message-ID: <08939576-e2b2-4b9f-ad72-a4f777d43a46@redhat.com> Date: Tue, 5 May 2026 19:08:07 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: intel_iommu unit test is also failing To: "Michael S. Tsirkin" Cc: =?UTF-8?Q?Cl=C3=A9ment_MATHIEU--DRIF?= , Peter Xu , Paolo Bonzini , "kvm@vger.kernel.org" , Yi Liu References: <96f57df07e6d39e30557357142b2212e0ea26af4.camel@bull.com> <0abf41c113c9425ea4c73a108db22f28290fa395.camel@bull.com> <13002aef21dec62205c252f3d12bb42ea59cf287.camel@bull.com> <6b338140-873c-4303-bdd1-633d69f4a971@redhat.com> <20260505061927-mutt-send-email-mst@kernel.org> <20260505065122-mutt-send-email-mst@kernel.org> <48384da2-7eb5-4044-bc3a-201b09d9cf81@redhat.com> <20260505081337-mutt-send-email-mst@kernel.org> Content-Language: en-US From: Thomas Huth Autocrypt: addr=thuth@redhat.com; keydata= xsFNBFH7eUwBEACzyOXKU+5Pcs6wNpKzrlJwzRl3VGZt95VCdb+FgoU9g11m7FWcOafrVRwU yYkTm9+7zBUc0sW5AuPGR/dp3pSLX/yFWsA/UB4nJsHqgDvDU7BImSeiTrnpMOTXb7Arw2a2 4CflIyFqjCpfDM4MuTmzTjXq4Uov1giGE9X6viNo1pxyEpd7PanlKNnf4PqEQp06X4IgUacW tSGj6Gcns1bCuHV8OPWLkf4hkRnu8hdL6i60Yxz4E6TqlrpxsfYwLXgEeswPHOA6Mn4Cso9O 0lewVYfFfsmokfAVMKWzOl1Sr0KGI5T9CpmRfAiSHpthhHWnECcJFwl72NTi6kUcUzG4se81 O6n9d/kTj7pzTmBdfwuOZ0YUSqcqs0W+l1NcASSYZQaDoD3/SLk+nqVeCBB4OnYOGhgmIHNW 0CwMRO/GK+20alxzk//V9GmIM2ACElbfF8+Uug3pqiHkVnKqM7W9/S1NH2qmxB6zMiJUHlTH gnVeZX0dgH27mzstcF786uPcdEqS0KJuxh2kk5IvUSL3Qn3ZgmgdxBMyCPciD/1cb7/Ahazr 3ThHQXSHXkH/aDXdfLsKVuwDzHLVSkdSnZdt5HHh75/NFHxwaTlydgfHmFFwodK8y/TjyiGZ zg2Kje38xnz8zKn9iesFBCcONXS7txENTzX0z80WKBhK+XSFJwARAQABzR5UaG9tYXMgSHV0 aCA8dGh1dGhAcmVkaGF0LmNvbT7CwXgEEwECACIFAlVgX6oCGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAAoJEC7Z13T+cC21EbIP/ii9cvT2HHGbFRl8HqGT6+7Wkb+XLMqJBMAIGiQK QIP3xk1HPTsLfVG0ao4hy/oYkGNOP8+ubLnZen6Yq3zAFiMhQ44lvgigDYJo3Ve59gfe99KX EbtB+X95ODARkq0McR6OAsPNJ7gpEUzfkQUUJTXRDQXfG/FX303Gvk+YU0spm2tsIKPl6AmV 1CegDljzjycyfJbk418MQmMu2T82kjrkEofUO2a24ed3VGC0/Uz//XCR2ZTo+vBoBUQl41BD eFFtoCSrzo3yPFS+w5fkH9NT8ChdpSlbNS32NhYQhJtr9zjWyFRf0Zk+T/1P7ECn6gTEkp5k ofFIA4MFBc/fXbaDRtBmPB0N9pqTFApIUI4vuFPPO0JDrII9dLwZ6lO9EKiwuVlvr1wwzsgq zJTPBU3qHaUO4d/8G+gD7AL/6T4zi8Jo/GmjBsnYaTzbm94lf0CjXjsOX3seMhaE6WAZOQQG tZHAO1kAPWpaxne+wtgMKthyPLNwelLf+xzGvrIKvLX6QuLoWMnWldu22z2ICVnLQChlR9d6 WW8QFEpo/FK7omuS8KvvopFcOOdlbFMM8Y/8vBgVMSsK6fsYUhruny/PahprPbYGiNIhKqz7 UvgyZVl4pBFjTaz/SbimTk210vIlkDyy1WuS8Zsn0htv4+jQPgo9rqFE4mipJjy/iboDzsFN BFH7eUwBEAC2nzfUeeI8dv0C4qrfCPze6NkryUflEut9WwHhfXCLjtvCjnoGqFelH/PE9NF4 4VPSCdvD1SSmFVzu6T9qWdcwMSaC+e7G/z0/AhBfqTeosAF5XvKQlAb9ZPkdDr7YN0a1XDfa +NgA+JZB4ROyBZFFAwNHT+HCnyzy0v9Sh3BgJJwfpXHH2l3LfncvV8rgFv0bvdr70U+On2XH 5bApOyW1WpIG5KPJlDdzcQTyptOJ1dnEHfwnABEfzI3dNf63rlxsGouX/NFRRRNqkdClQR3K gCwciaXfZ7ir7fF0u1N2UuLsWA8Ei1JrNypk+MRxhbvdQC4tyZCZ8mVDk+QOK6pyK2f4rMf/ WmqxNTtAVmNuZIwnJdjRMMSs4W4w6N/bRvpqtykSqx7VXcgqtv6eqoDZrNuhGbekQA0sAnCJ VPArerAZGArm63o39me/bRUQeQVSxEBmg66yshF9HkcUPGVeC4B0TPwz+HFcVhheo6hoJjLq knFOPLRj+0h+ZL+D0GenyqD3CyuyeTT5dGcNU9qT74bdSr20k/CklvI7S9yoQje8BeQAHtdV cvO8XCLrpGuw9SgOS7OP5oI26a0548M4KldAY+kqX6XVphEw3/6U1KTf7WxW5zYLTtadjISB X9xsRWSU+Yqs3C7oN5TIPSoj9tXMoxZkCIHWvnqGwZ7JhwARAQABwsFfBBgBAgAJBQJR+3lM AhsMAAoJEC7Z13T+cC21hPAQAIsBL9MdGpdEpvXs9CYrBkd6tS9mbaSWj6XBDfA1AEdQkBOn ZH1Qt7HJesk+qNSnLv6+jP4VwqK5AFMrKJ6IjE7jqgzGxtcZnvSjeDGPF1h2CKZQPpTw890k fy18AvgFHkVk2Oylyexw3aOBsXg6ukN44vIFqPoc+YSU0+0QIdYJp/XFsgWxnFIMYwDpxSHS 5fdDxUjsk3UBHZx+IhFjs2siVZi5wnHIqM7eK9abr2cK2weInTBwXwqVWjsXZ4tq5+jQrwDK cvxIcwXdUTLGxc4/Z/VRH1PZSvfQxdxMGmNTGaXVNfdFZjm4fz0mz+OUi6AHC4CZpwnsliGV ODqwX8Y1zic9viSTbKS01ZNp175POyWViUk9qisPZB7ypfSIVSEULrL347qY/hm9ahhqmn17 Ng255syASv3ehvX7iwWDfzXbA0/TVaqwa1YIkec+/8miicV0zMP9siRcYQkyTqSzaTFBBmqD oiT+z+/E59qj/EKfyce3sbC9XLjXv3mHMrq1tKX4G7IJGnS989E/fg6crv6NHae9Ckm7+lSs IQu4bBP2GxiRQ+NV3iV/KU3ebMRzqIC//DCOxzQNFNJAKldPe/bKZMCxEqtVoRkuJtNdp/5a yXFZ6TfE1hGKrDBYAm4vrnZ4CXFSBDllL59cFFOJCkn4Xboj/aVxxJxF30bn In-Reply-To: <20260505081337-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 05/05/2026 14.33, Michael S. Tsirkin wrote: > 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? > > So then it's this bug apparently: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125180 Ah, thanks, that looks like the bug, indeed! > will likely be in 16.2? > > Maybe just wait? I guess we could drop the failing test from the CI for the time being... Thomas