Re: [PATCH V4 04/50] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()

[Lai Jiangshan]

On 2021/11/2 16:58, Borislav Petkov wrote:

> >   	 */
> >   
> > -	ALTERNATIVE "", "jmp	swapgs_restore_regs_and_return_to_usermode", \
> > +	ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", \
>
> Instead of sprinkling all those ALTERNATIVE calls everywhere,
> why don't you simply jump to the xenpv-one at the
> swapgs_restore_regs_and_return_to_usermode label itself and have a
> single ALTERNATIVE there?

It will add a 5-byte NOP at the beginning of the native
swapgs_restore_regs_and_return_to_usermode.  I avoided adding unneeded
code in the native code even if it is NOPs and avoided melting xenpv-one into
the native one which will reduce the code readability.

I will follow your preference since a 5-byte NOP is so negligible in the slow
path with an iret instruction.

Or other option that adds macros to wrap the ALTERNATIVE.
RESTORE_REGS_AND_RETURN_TO_USERMODE and
COND_RESTORE_REGS_AND_RETURN_TO_USERMODE (test %eax before jmp in native case)