Re: [Xen-devel] [PATCH RFC for-4.13 04/10] xen/arm: Ensure the SSBD workaround is re-enabled right after exiting a guest

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 27/09/2019 15:21, Volodymyr Babchuk wrote:
> Julien Grall writes:
>
> > On 27/09/2019 14:33, Volodymyr Babchuk wrote:
> > > Julien Grall writes:
> > > > On 27/09/2019 13:39, Volodymyr Babchuk wrote:
> > > > > Julien Grall writes:
> > > > > > On 27/09/2019 12:56, Volodymyr Babchuk wrote:
> > > > > > > Julien Grall writes:
> > > > > Or maybe, we should not split the function at all? Instead, we enable
> > > > > interrupts right in the middle of it.
> > > > I thought about this but I didn't much like the resulting code.
> > > >
> > > > The instruction to unmask interrupts requires to take an immediate
> > > > (indicates which interrupts to unmask). As not all the traps require
> > > > to unmask the same interrupts, we would end up to have to a bunch of
> > > > if in the code to select the right unmasking.
> > > Ah, yes, this is the problem. We can provide callback to
> > > enter_hypervisor_from_guest().
> > I am not sure what you mean by this. Do you mean a callback that will
> > unmask the interrupts?
> Yes. You can pass function pointer to enter_hypervisor_from_guest(). To
> a function, that will unmask the interrupts. I'm sure that guest_vector
> macro can generate it for you. Something like this:
>
>          .macro  guest_vector compat, iflags, trap, save_x0_x1=1
>          entry   hyp=0, compat=\compat, save_x0_x1=\save_x0_x1
>          /*
>           * The vSError will be checked while SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT
>           * is not set. If a vSError took place, the initial exception will be
>           * skipped. Exit ASAP
>           */
>          ALTERNATIVE("bl check_pending_vserror; cbnz x0, 1f",
>                      "nop; nop",
>                      SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT)
>          ldr     x0, =1f
>          bl      enter_hypervisor_from_guest
>          mov     x0, sp
>          bl      do_trap_\trap
>          b       1f
> 2:
>          msr     daifclr, \iflags
>          ret
> 1:
>          exit    hyp=0, compat=\compat
>          .endm
TBH, I don't see what's the point you are trying to make here. Yes, there are 
many way to write a code and possibility to make one function. You could also 
create a skeleton macro for enter_hypervisor_from_guest and generate N of them 
(one per set of unmask interrupts) and call them.
But are they really worth it?

Cheers,

--
Julien Grall

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