Re: [PATCH v2 04/12] x86/xen: drop USERGS_SYSRET64 paravirt call

[Jürgen Groß]

On 02.12.20 13:32, Borislav Petkov wrote:
> On Fri, Nov 20, 2020 at 12:46:22PM +0100, Juergen Gross wrote:
> > @@ -123,12 +115,15 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
> >   	 * Try to use SYSRET instead of IRET if we're returning to
> >   	 * a completely clean 64-bit userspace context.  If we're not,
> >   	 * go to the slow exit path.
> > +	 * In the Xen PV case we must use iret anyway.
> >   	 */
> > -	movq	RCX(%rsp), %rcx
> > -	movq	RIP(%rsp), %r11
> >   
> > -	cmpq	%rcx, %r11	/* SYSRET requires RCX == RIP */
> > -	jne	swapgs_restore_regs_and_return_to_usermode
> > +	ALTERNATIVE __stringify( \
> > +		movq	RCX(%rsp), %rcx; \
> > +		movq	RIP(%rsp), %r11; \
> > +		cmpq	%rcx, %r11;	/* SYSRET requires RCX == RIP */ \
> > +		jne	swapgs_restore_regs_and_return_to_usermode), \
> > +	"jmp	swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
>
> Why such a big ALTERNATIVE when you can simply do:
>
>          /*
>           * Try to use SYSRET instead of IRET if we're returning to
>           * a completely clean 64-bit userspace context.  If we're not,
>           * go to the slow exit path.
>           * In the Xen PV case we must use iret anyway.
>           */
>          ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
>
>          movq    RCX(%rsp), %rcx;
>          movq    RIP(%rsp), %r11;
>          cmpq    %rcx, %r11;     /* SYSRET requires RCX == RIP */ \
>          jne     swapgs_restore_regs_and_return_to_usermode
>
> ?

I wanted to avoid the additional NOPs for the bare metal case.

If you don't mind them I can do as you are suggesting.


Juergen

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