Re: [kvm-unit-tests PATCH 1/2] x86: Do not assign values to unaligned pointer to 128 bits

[Jacob Xu <jacobhxu@google.com>]

ping (previous ping just now contained html subpart, oops).


On Mon, May 10, 2021 at 6:50 PM Jacob Xu <jacobhxu@google.com> 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 sse_union, we get a GP when
> running. Let's avoid using a pointer to sse_union at all, since doing so
> implies that 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 | 67 +++++++++++++++++++++++++-------------------------
>  1 file changed, 33 insertions(+), 34 deletions(-)
>
> diff --git a/x86/emulator.c b/x86/emulator.c
> index 9705073..1d5c172 100644
> --- a/x86/emulator.c
> +++ b/x86/emulator.c
> @@ -645,37 +645,34 @@ static void test_muldiv(long *mem)
>
>  typedef unsigned __attribute__((vector_size(16))) sse128;
>
> -typedef union {
> -    sse128 sse;
> -    unsigned u[4];
> -} sse_union;
> -
> -static bool sseeq(sse_union *v1, sse_union *v2)
> +static bool sseeq(uint32_t *v1, uint32_t *v2)
>  {
>      bool ok = true;
>      int i;
>
>      for (i = 0; i < 4; ++i) {
> -       ok &= v1->u[i] == v2->u[i];
> +       ok &= v1[i] == v2[i];
>      }
>
>      return ok;
>  }
>
> -static __attribute__((target("sse2"))) void test_sse(sse_union *mem)
> +static __attribute__((target("sse2"))) void test_sse(uint32_t *mem)
>  {
> -       sse_union v;
> +       sse128 vv;
> +       uint32_t *v = (uint32_t *)&vv;
>
>         write_cr0(read_cr0() & ~6); /* EM, TS */
>         write_cr4(read_cr4() | 0x200); /* OSFXSR */
> +       memset(&vv, 0, sizeof(vv));
>
>  #define TEST_RW_SSE(insn) do { \
> -               v.u[0] = 1; v.u[1] = 2; v.u[2] = 3; v.u[3] = 4; \
> -               asm(insn " %1, %0" : "=m"(*mem) : "x"(v.sse)); \
> -               report(sseeq(&v, mem), insn " (read)"); \
> -               mem->u[0] = 5; mem->u[1] = 6; mem->u[2] = 7; mem->u[3] = 8; \
> -               asm(insn " %1, %0" : "=x"(v.sse) : "m"(*mem)); \
> -               report(sseeq(&v, mem), insn " (write)"); \
> +               v[0] = 1; v[1] = 2; v[2] = 3; v[3] = 4; \
> +               asm(insn " %1, %0" : "=m"(*mem) : "x"(vv) : "memory"); \
> +               report(sseeq(v, mem), insn " (read)"); \
> +               mem[0] = 5; mem[1] = 6; mem[2] = 7; mem[3] = 8; \
> +               asm(insn " %1, %0" : "=x"(vv) : "m"(*mem) : "memory"); \
> +               report(sseeq(v, mem), insn " (write)"); \
>  } while (0)
>
>         TEST_RW_SSE("movdqu");
> @@ -704,40 +701,41 @@ static void cross_movups_handler(struct ex_regs *regs)
>
>  static __attribute__((target("sse2"))) void test_sse_exceptions(void *cross_mem)
>  {
> -       sse_union v;
> -       sse_union *mem;
> +       sse128 vv;
> +       uint32_t *v = (uint32_t *)&vv;
> +       uint32_t *mem;
>         uint8_t *bytes = cross_mem; // aligned on PAGE_SIZE*2
>         void *page2 = (void *)(&bytes[4096]);
>         struct pte_search search;
>         pteval_t orig_pte;
>
>         // setup memory for unaligned access
> -       mem = (sse_union *)(&bytes[8]);
> +       mem = (uint32_t *)(&bytes[8]);
>
>         // 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;
> -       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;
> -       asm("movupd %1, %0" : "=m"(*mem) : "x"(v.sse));
> -       report(sseeq(&v, mem), "movupd unaligned");
> +       v[0] = 1; v[1] = 2; v[2] = 3; v[3] = 4;
> +       mem[0] = 5; mem[1] = 6; mem[2] = 8; mem[3] = 9;
> +       asm("movups %1, %0" : "=m"(*mem) : "x"(vv) : "memory");
> +       report(sseeq(v, mem), "movups unaligned");
> +
> +       v[0] = 1; v[1] = 2; v[2] = 3; v[3] = 4;
> +       mem[0] = 5; mem[1] = 6; mem[2] = 7; mem[3] = 8;
> +       asm("movupd %1, %0" : "=m"(*mem) : "x"(vv) : "memory");
> +       report(sseeq(v, mem), "movupd unaligned");
>         exceptions = 0;
>         handle_exception(GP_VECTOR, unaligned_movaps_handler);
>         asm("movaps %1, %0\n\t unaligned_movaps_cont:"
> -                       : "=m"(*mem) : "x"(v.sse));
> +                       : "=m"(*mem) : "x"(vv));
>         handle_exception(GP_VECTOR, 0);
>         report(exceptions == 1, "unaligned movaps exception");
>
>         // 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;
> +       mem = (uint32_t *)(&bytes[4096-8]);
> +       v[0] = 1; v[1] = 2; v[2] = 3; v[3] = 4;
> +       mem[0] = 5; mem[1] = 6; mem[2] = 7; mem[3] = 8;
>
> -       asm("movups %1, %0" : "=m"(*mem) : "x"(v.sse));
> -       report(sseeq(&v, mem), "movups unaligned crosspage");
> +       asm("movups %1, %0" : "=m"(*mem) : "x"(vv) : "memory");
> +       report(sseeq(v, mem), "movups unaligned crosspage");
>
>         // invalidate second page
>         search = find_pte_level(current_page_table(), page2, 1);
> @@ -747,7 +745,8 @@ static __attribute__((target("sse2"))) void test_sse_exceptions(void *cross_mem)
>
>         exceptions = 0;
>         handle_exception(PF_VECTOR, cross_movups_handler);
> -       asm("movups %1, %0\n\t cross_movups_cont:" : "=m"(*mem) : "x"(v.sse));
> +       asm("movups %1, %0\n\t cross_movups_cont:" : "=m"(*mem) : "x"(vv) :
> +                       "memory");
>         handle_exception(PF_VECTOR, 0);
>         report(exceptions == 1, "movups crosspage exception");
>
> --
> 2.31.1.607.g51e8a6a459-goog
>