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* [PATCH] x86: Do not assign values to unaligned pointer to 128 bits
@ 2021-05-06  0:48 Jacob Xu
  2021-05-06 16:01 ` Sean Christopherson
  0 siblings, 1 reply; 4+ messages in thread
From: Jacob Xu @ 2021-05-06  0:48 UTC (permalink / raw)
  To: Paolo Bonzini, Jim Mattson; +Cc: kvm, Jacob Xu

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
uint8_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..672bfda 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, 0, 128);
 	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, 0, 128);
 	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, 0, 128);
 
 	asm("movups %1, %0" : "=m"(*mem) : "x"(v.sse));
 	report(sseeq(&v, mem), "movups unaligned crosspage");
-- 
2.31.1.527.g47e6f16901-goog


^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: [PATCH] x86: Do not assign values to unaligned pointer to 128 bits
  2021-05-06  0:48 [PATCH] x86: Do not assign values to unaligned pointer to 128 bits Jacob Xu
@ 2021-05-06 16:01 ` Sean Christopherson
  2021-05-06 16:40   ` Jim Mattson
  0 siblings, 1 reply; 4+ messages in thread
From: Sean Christopherson @ 2021-05-06 16:01 UTC (permalink / raw)
  To: Jacob Xu; +Cc: Paolo Bonzini, Jim Mattson, kvm

Please use [kvm-unit-tests PATCH ...] for the subject, it took me a depressingly
long time to figure out which code base this applied to (though admittedly there
was a non-zero amount of PEBKAC going on).

On Wed, May 05, 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
> uint8_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..672bfda 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, 0, 128);

Shouldn't this be '16', as in 16 bytes / 128 bits?  And would it makes sense to
use a pattern other than '0', if only for giggles?

>  	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, 0, 128);
>  	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, 0, 128);
>  
>  	asm("movups %1, %0" : "=m"(*mem) : "x"(v.sse));
>  	report(sseeq(&v, mem), "movups unaligned crosspage");
> -- 
> 2.31.1.527.g47e6f16901-goog
> 

^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: [PATCH] x86: Do not assign values to unaligned pointer to 128 bits
  2021-05-06 16:01 ` Sean Christopherson
@ 2021-05-06 16:40   ` Jim Mattson
  2021-05-06 18:50     ` Jacob Xu
  0 siblings, 1 reply; 4+ messages in thread
From: Jim Mattson @ 2021-05-06 16:40 UTC (permalink / raw)
  To: Sean Christopherson; +Cc: Jacob Xu, Paolo Bonzini, kvm list

On Thu, May 6, 2021 at 9:01 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Please use [kvm-unit-tests PATCH ...] for the subject, it took me a depressingly
> long time to figure out which code base this applied to (though admittedly there
> was a non-zero amount of PEBKAC going on).
>
> On Wed, May 05, 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
> > uint8_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..672bfda 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, 0, 128);
>
> Shouldn't this be '16', as in 16 bytes / 128 bits?  And would it makes sense to
> use a pattern other than '0', if only for giggles?

Or possibly sizeof(*mem)?

^ permalink raw reply	[flat|nested] 4+ messages in thread

* Re: [PATCH] x86: Do not assign values to unaligned pointer to 128 bits
  2021-05-06 16:40   ` Jim Mattson
@ 2021-05-06 18:50     ` Jacob Xu
  0 siblings, 0 replies; 4+ messages in thread
From: Jacob Xu @ 2021-05-06 18:50 UTC (permalink / raw)
  To: Jim Mattson; +Cc: Sean Christopherson, Paolo Bonzini, kvm list

> Please use [kvm-unit-tests PATCH ...] for the subject
Oops, I'll resend v2 with the correct prefix.

> Shouldn't this be '16', as in 16 bytes / 128 bits?
> Or possibly sizeof(*mem)?
Replaced with sizeof below.

> use a pattern other than '0', if only for giggles?
replaced uint8_t with uint32_t for more giggles and selected
0xdecafbad from the wikipedia page for Hexspeak.


>  And would it makes sense to use a pattern other than '0', if only for giggles?


> Or possibly sizeof(*mem)?

^ permalink raw reply	[flat|nested] 4+ messages in thread

end of thread, other threads:[~2021-05-06 18:50 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2021-05-06  0:48 [PATCH] x86: Do not assign values to unaligned pointer to 128 bits Jacob Xu
2021-05-06 16:01 ` Sean Christopherson
2021-05-06 16:40   ` Jim Mattson
2021-05-06 18:50     ` Jacob Xu

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