Re: [PATCH v4 07/30] xen/asm-generic: introdure nospec.h

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

Hi Julien,

On Sun, 2024-02-18 at 18:30 +0000, Julien Grall wrote:
> Hi Oleksii,
> 
> Title: Typo s/introdure/introduce/
> 
> On 05/02/2024 15:32, Oleksii Kurochko wrote:
> > The <asm/nospec.h> header is similar between Arm, PPC, and RISC-V,
> > so it has been moved to asm-generic.
> 
> I am not 100% convinced that moving this header to asm-generic is a
> good 
> idea. At least for Arm, those helpers ought to be non-empty, what
> about 
> RISC-V?
For Arm, they are not taking any action, are they? There are no
specific fences or other mechanisms inside
evaluate_nospec()/block_speculation() to address speculation.

For RISC-V, it can be implemented in a similar manner, at least for
now. Since these functions are only used in the grant tables code ( for
Arm and so for RISC-V ), which is not supported by RISC-V.

> 
> If the answer is they should be non-empty. Then I would consider to
> keep 
> the duplication to make clear that each architecture should take
> their 
> own decision in term of security.
> 
> The alternative, is to have a generic implementation that is safe by 
> default (if that's even possible).
I am not certain that we can have a generic implementation, as each
architecture may have specific speculation issues.

~ Oleksii