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

[Jacob Xu <jacobhxu@google.com>]

> memset() takes a void *, which it casts to an char, i.e. it works on one byte at
a time.
Huh, TIL. Based on this I'd thought that I don't need a cast at all,
but doing so actually results in a movaps instruction.
I've changed the cast back to (uint8_t *).

> The size needs to be sizeof(*mem)
Bah, thanks for catching that.

I've updated it below.
---
 x86/emulator.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/x86/emulator.c b/x86/emulator.c
index 9705073..ea23ef1 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((uint8_t *)mem, 5, sizeof(*mem));
        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((uint8_t *)mem, 5, 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((uint8_t *)mem, 5, sizeof(*mem));

        asm("movups %1, %0" : "=m"(*mem) : "x"(v.sse));
        report(sseeq(&v, mem), "movups unaligned crosspage");
--


On Thu, May 6, 2021 at 11:58 AM Sean Christopherson <seanjc@google.com> wrote:
>
> 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
> >