On Mon 12.Oct'20 at 12:49:16 -0400, Arvind Sankar wrote:
On Mon, Oct 12, 2020 at 04:44:31PM +0800, Shuo A Liu wrote:On Wed 30.Sep'20 at 12:14:03 -0700, Nick Desaulniers wrote: >On Wed, Sep 30, 2020 at 10:13 AM Peter Zijlstra <[email protected]> wrote: >> >> On Wed, Sep 30, 2020 at 11:10:36AM -0500, Segher Boessenkool wrote: >> >> > Since this variable is a local register asm, on entry to the asm the >> > compiler guarantees that the value lives in the assigned register (the >> > "r8" hardware register in this case). This all works completely fine. >> > This is the only guaranteed behaviour for local register asm (well, >> > together with analogous behaviour for outputs). >> >> Right, that's what they're trying to achieve. The hypervisor calling >> convention needs that variable in %r8 (which is somewhat unfortunate). >> >> AFAIK this is the first such use in the kernel, but at least the gcc-4.9 >> (our oldest supported version) claims to support this. >> >> So now we need to know if clang will actually do this too.. > >Does clang support register local storage? Let's use godbolt.org to find out: >https://godbolt.org/z/YM45W5 >Looks like yes. You can even check different GCC versions via the >dropdown in the top right. > >The -ffixed-* flags are less well supported in Clang; they need to be >reimplemented on a per-backend basis. aarch64 is relatively well >supported, but other arches not so much IME. > >Do we need register local storage here? > >static inline long bar(unsigned long hcall_id) >{ > long result; > asm volatile("movl %1, %%r8d\n\t" > "vmcall\n\t" > : "=a" (result) > : "ir" (hcall_id) > : ); > return result; >}Yeah, this approach is also mentioned in the changelog. I will change to this way to follow your preference. With an addtional "r8" clobber what Arvind mentioned. Thanks shuoBtw, I noticed that arch/x86/xen/hypercall.h uses register-local variables already for its hypercalls for quite some time, so this wouldn't be unprecedented. [0] Do these calls also need a memory clobber? The KVM/xen hypercall functions all have one.
Yes. it's needed. I will add it. Thanks
Thanks.
[0] e74359028d548 ("xen64: fix calls into hypercall page")
