Re: [RFC][PATCH 15/22] x86,vmx: Remove .fixup usage

Peter Zijlstra
2021-11-05T18:54:53Z

On Fri, Nov 05, 2021 at 06:17:53PM +0000, Sean Christopherson wrote:
> On Thu, Nov 04, 2021, Peter Zijlstra wrote:
> > In the vmread exceptin path, use the, thus far, unused output register
> > to push the @fault argument onto the stack. This, in turn, enables the
> > exception handler to not do pushes and only modify that register when
> > an exception does occur.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> > arch/x86/kvm/vmx/vmx_ops.h | 14 ++++++--------
> > 1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > --- a/arch/x86/kvm/vmx/vmx_ops.h
> > +++ b/arch/x86/kvm/vmx/vmx_ops.h
> > @@ -80,9 +80,11 @@ static __always_inline unsigned long __v
> > * @field, and bounce through the trampoline to preserve
> > * volatile registers.
> > */
> > - "push $0\n\t"
> > + "xorl %k1, %k1\n\t"
> > + "2:\n\t"
> > + "push %1\n\t"
> > "push %2\n\t"
>
> This trick doesn't work if the compiler selects the same GPR for %1 and %2, as
> the "field" will get lost.
>
> 0x00000000000005a2 <+66>: 0f 78 c0 vmread %rax,%rax
> 0x00000000000005a5 <+69>: 3e 77 0b ja,pt 0x5b3 <vmx_read_guest_seg_selector+83>
> 0x00000000000005a8 <+72>: 31 c0 xor %eax,%eax
> 0x00000000000005aa <+74>: 50 push %rax
> 0x00000000000005ab <+75>: 50 push %rax

*argh* I misread that thing, %2 is an input reg :/ Thanks for your
patch.