On Wed, 2023-06-28 at 17:29 +0200, Peter Zijlstra wrote:
> On Tue, Jun 27, 2023 at 02:12:50AM +1200, Kai Huang wrote:
> > diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
> > index 49a54356ae99..757b0c34be10 100644
> > --- a/arch/x86/virt/vmx/tdx/tdxcall.S
> > +++ b/arch/x86/virt/vmx/tdx/tdxcall.S
> > @@ -1,6 +1,7 @@
> > /* SPDX-License-Identifier: GPL-2.0 */
> > #include <asm/asm-offsets.h>
> > #include <asm/tdx.h>
> > +#include <asm/asm.h>
> >
> > /*
> > * TDCALL and SEAMCALL are supported in Binutils >= 2.36.
> > @@ -45,6 +46,7 @@
> > /* Leave input param 2 in RDX */
> >
> > .if \host
> > +1:
> > seamcall
>
> So what registers are actually clobbered by SEAMCALL ? There's a
> distinct lack of it in SDM Vol.2 instruction list :-(
>
> > /*
> > * SEAMCALL instruction is essentially a VMExit from VMX root
> > @@ -57,10 +59,23 @@
> > * This value will never be used as actual SEAMCALL error code as
> > * it is from the Reserved status code class.
> > */
> > - jnc .Lno_vmfailinvalid
> > + jnc .Lseamcall_out
> > mov $TDX_SEAMCALL_VMFAILINVALID, %rax
> > -.Lno_vmfailinvalid:
> > + jmp .Lseamcall_out
> > +2:
> > + /*
> > + * SEAMCALL caused #GP or #UD. By reaching here %eax contains
> > + * the trap number. Convert the trap number to the TDX error
> > + * code by setting TDX_SW_ERROR to the high 32-bits of %rax.
> > + *
> > + * Note cannot OR TDX_SW_ERROR directly to %rax as OR instruction
> > + * only accepts 32-bit immediate at most.
> > + */
> > + mov $TDX_SW_ERROR, %r12
> > + orq %r12, %rax
> >
> > + _ASM_EXTABLE_FAULT(1b, 2b)
> > +.Lseamcall_out:
>
> This is all pretty atrocious code flow... would it at all be possible to
> write it like:
>
> SYM_FUNC_START(...)
>
> .if \host
> 1: seamcall
> cmovc %spare, %rax
Looks using cmovc can remove the using of one additional label.
I guess it can be done in a separate patch.
> 2:
> .else
> tdcall
> .endif
>
> .....
> RET
>
>
> 3:
> mov $TDX_SW_ERROR, %r12
> orq %r12, %rax
> jmp 2b
>
> _ASM_EXTABLE_FAULT(1b, 3b)
>
> SYM_FUNC_END()
>
> That is, having all that inline in the hotpath is quite horrific.
>
The __tdx_module_call() and __seamcall() both has a "RET" at the end of this
macro:
SYM_FUNC_START(__tdx_module_call)
FRAME_BEGIN
TDX_MODULE_CALL host=0
FRAME_END
RET
SYM_FUNC_END(__tdx_module_call)
In this case, I think we need to remove the "RET" there.
I didn't think I would modify __tdx_module_call() and __seamcall() in this
patch.
