Re: [PATCH 2/2] xen/arm: Support runstate crossing pages

[Julien Grall <julien@xxxxxxx>]

Hi,

On 12/06/2020 02:10, Stefano Stabellini wrote:
> On Thu, 11 Jun 2020, Bertrand Marquis wrote:
> > Add support for runstate area register with the structure crossing pages
> > The code is storing up to 2 pages reference during the hypercall.
> > During a context switch, the code is computing where the
> > state_entry_time is and is breaking the memcpy in 2 parts when it is
> > required.
> >
> > Signed-off-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
> Clearly a lot of efforts went into this patch, thanks you Bertrand.
>
> The change is complex for the feature it adds. I wonder if we could use
> ioremap (or maybe something similar but using a different virtual space)
> to simplify it. Julien, do you have good ideas?
ioremap() is for MMIO region, so you would want to use vmap(). Note that 
former is just a wrapper of the latter.
vmap() is probably a good start for now, but not ideal for long term 
because the vmap() area is fairly small (768MB) and if we want to go 
towards a secret-free hypervisor on Arm, we would want to restrict the 
visibility of the mapping to the other CPUs.
I think it would be good to have some per-CPU/per-domain mapping to 
limit the waste of the address space. But this will require to 
deduplicate page-tables for arm64 (I was actually looking at it over the 
past few week-ends).
For the time-being, I would suggest to use vmap(). The memory is always 
direct mapped on arm64, so it should make no different for security concern.
Cheers,

--
Julien Grall