Peter Zijlstra
2017-Feb-16 16:09 UTC
[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.
Waiman Long
2017-Feb-16 21:02 UTC
[PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long
On 02/16/2017 11:09 AM, Peter Zijlstra wrote:> 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.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
Peter Zijlstra
2017-Feb-17 09:42 UTC
[PATCH v4 1/2] x86/paravirt: Change vcp_is_preempted() arg type to long
On Thu, Feb 16, 2017 at 04:02:57PM -0500, Waiman Long wrote:> On 02/16/2017 11:09 AM, Peter Zijlstra wrote: > > 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. > > 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.
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