Re: [PATCH] xen/arm64: Zero the top 32 bits of gp registers on entry...

[Julien Grall <julien@xxxxxxx>]

Hi Michal,

On 15/12/2021 10:40, Michal Orzel wrote:
> On 15.12.2021 11:32, Jan Beulich wrote:
> > (Re-sending an abridged version, as apparently spam filters didn't like
> > the original message with more retained context; I'll have to see whether
> > this one also isn't liked. Sorry.)
> >
> > On 15.12.2021 10:48, Michal Orzel wrote:
> > > This patch and the problem it solves is about clearing top 32bits of all gp 
> > > registers so not only x0,x1.
> > That's well understood. Yet for everything still in registers simply
> > using mov ahead of the respective push (as you had it) is still
> > preferable imo.
> >
> > Jan
> In that case let's wait for Julien's opinion to decide whether I should get 
> back to the previous
> solution with mov or to the stack solution.
IIUC, your proposal is to:
   1) Push all the 64-bit registers
   2) Zero the top 32-bit

Jan's suggestion is to:
   1) clobber the top 32-bit using mov wX, wX
   2) Push all the registers

My preference is for the latter because there will be less memory/cache 
access.
So, this would be your original patch + a compile time check to ensure 
save_x0_x1 is 0 when compat=1.
Cheers,

--
Julien Grall