Re: [kvm-unit-tests PATCH v2] x86: Do not assign values to unaligned pointer to 128 bits

[Sean Christopherson <seanjc@google.com>]

On Thu, May 06, 2021, Jacob Xu wrote:
> When compiled with clang, the following statement gets converted into a
> movaps instructions.
> mem->u[0] = 5; mem->u[1] = 6; mem->u[2] = 7; mem->u[3] = 8;
> 
> Since mem is an unaligned pointer to a union of an sse, we get a GP when
> running.
> 
> All we want is to make the values between mem and v different for this
> testcase, so let's just memset the pointer at mem, and convert to
> uint32_t pointer. Then the compiler will not assume the pointer is
> aligned to 128 bits.
> 
> Fixes: e5e76263b5 ("x86: add additional test cases for sse exceptions to
> emulator.c")
> 
> Signed-off-by: Jacob Xu <jacobhxu@google.com>
> ---
>  x86/emulator.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/x86/emulator.c b/x86/emulator.c
> index 9705073..a2c7e5b 100644
> --- a/x86/emulator.c
> +++ b/x86/emulator.c
> @@ -716,12 +716,12 @@ static __attribute__((target("sse2"))) void test_sse_exceptions(void *cross_mem)
>  
>  	// test unaligned access for movups, movupd and movaps
>  	v.u[0] = 1; v.u[1] = 2; v.u[2] = 3; v.u[3] = 4;
> -	mem->u[0] = 5; mem->u[1] = 6; mem->u[2] = 7; mem->u[3] = 8;
> +	memset((uint32_t *)mem, 0xdecafbad, sizeof(mem));

memset() takes a void *, which it casts to an char, i.e. it works on one byte at
a time.  Casting to a uint32_t won't make it write the full "0xdecafbad", it will
just repease 0xad over and over.

The size needs to be sizeof(*mem), i.e. the size of the object that mem points to,
not the size of the pointer's storage.

>  	asm("movups %1, %0" : "=m"(*mem) : "x"(v.sse));
>  	report(sseeq(&v, mem), "movups unaligned");
>  
>  	v.u[0] = 1; v.u[1] = 2; v.u[2] = 3; v.u[3] = 4;
> -	mem->u[0] = 5; mem->u[1] = 6; mem->u[2] = 7; mem->u[3] = 8;
> +	memset((uint32_t *)mem, 0xdecafbad, sizeof(mem));
>  	asm("movupd %1, %0" : "=m"(*mem) : "x"(v.sse));
>  	report(sseeq(&v, mem), "movupd unaligned");
>  	exceptions = 0;
> @@ -734,7 +734,7 @@ static __attribute__((target("sse2"))) void test_sse_exceptions(void *cross_mem)
>  	// setup memory for cross page access
>  	mem = (sse_union *)(&bytes[4096-8]);
>  	v.u[0] = 1; v.u[1] = 2; v.u[2] = 3; v.u[3] = 4;
> -	mem->u[0] = 5; mem->u[1] = 6; mem->u[2] = 7; mem->u[3] = 8;
> +	memset((uint32_t *)mem, 0xdecafbad, sizeof(mem));
>  
>  	asm("movups %1, %0" : "=m"(*mem) : "x"(v.sse));
>  	report(sseeq(&v, mem), "movups unaligned crosspage");
> -- 
> 2.31.1.607.g51e8a6a459-goog
>