Re: [PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long

On Wed, Feb 15, 2017 at 04:37:49PM -0500, Waiman Long wrote:

The cpu argument in the function prototype of vcpu_is_preempted()
is changed from int to long. That makes it easier to provide a better
optimized assembly version of that function.

For Xen, vcpu_is_preempted(long) calls xen_vcpu_stolen(int), the
downcast from long to int is not a problem as vCPU number won't exceed
32 bits.

Note that because of the cast in PVOP_CALL_ARG1() this patch is
pointless.

Then again, it doesn't seem to affect code generation, so why not. Takes
away the reliance on that weird cast.

On 02/16/2017 11:09 AM, Peter Zijlstra wrote:

On Wed, Feb 15, 2017 at 04:37:49PM -0500, Waiman Long wrote:

quoted

The cpu argument in the function prototype of vcpu_is_preempted()
is changed from int to long. That makes it easier to provide a better
optimized assembly version of that function.

For Xen, vcpu_is_preempted(long) calls xen_vcpu_stolen(int), the
downcast from long to int is not a problem as vCPU number won't exceed
32 bits.

Note that because of the cast in PVOP_CALL_ARG1() this patch is
pointless.

Then again, it doesn't seem to affect code generation, so why not. Takes
away the reliance on that weird cast.

I add this patch because I am a bit uneasy about clearing the upper 32
bits of rdi and assuming that the compiler won't have a previous use of
those bits. It gives me peace of mind.

Cheers,
Longman

On Thu, Feb 16, 2017 at 04:02:57PM -0500, Waiman Long wrote:

On 02/16/2017 11:09 AM, Peter Zijlstra wrote:

quoted

On Wed, Feb 15, 2017 at 04:37:49PM -0500, Waiman Long wrote:

quoted

The cpu argument in the function prototype of vcpu_is_preempted()
is changed from int to long. That makes it easier to provide a better
optimized assembly version of that function.

For Xen, vcpu_is_preempted(long) calls xen_vcpu_stolen(int), the
downcast from long to int is not a problem as vCPU number won't exceed
32 bits.

Note that because of the cast in PVOP_CALL_ARG1() this patch is
pointless.

Then again, it doesn't seem to affect code generation, so why not. Takes
away the reliance on that weird cast.

I add this patch because I am a bit uneasy about clearing the upper 32
bits of rdi and assuming that the compiler won't have a previous use of
those bits. It gives me peace of mind.

So currently the PVOP_CALL_ARG#() macros force cast everything to
(unsigned long) anyway, but it would be good not to rely on that I
think, so yes.