Re: [PATCH][4.15] x86: mirror compat argument translation area for 32-bit PV

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.02.2021 15:13, Roger Pau Monné wrote:
> On Mon, Feb 22, 2021 at 12:35:21PM +0100, Roger Pau Monné wrote:
>> On Mon, Feb 22, 2021 at 11:27:07AM +0100, Jan Beulich wrote:
>>> Now that we guard the entire Xen VA space against speculative abuse
>>> through hypervisor accesses to guest memory, the argument translation
>>> area's VA also needs to live outside this range, at least for 32-bit PV
>>> guests. To avoid extra is_hvm_*() conditionals, use the alternative VA
>>> uniformly.
>>
>> Since you are double mapping the per-domain virtual area, won't it
>> make more sense to map it just once outside of the Xen virtual space
>> area? (so it's always using PML4_ADDR(511))
> 
> Right, that's not possible for PV 64bit domains because it's guest
> owned linear address space in that case.
> 
> It seems like paravirt_ctxt_switch_to will modify the root_pgt to set
> the PERDOMAIN_VIRT_START entry, does the same need to be done for
> PERDOMAIN2_VIRT_START?

I don't think so, no. Argument translation doesn't happen when
the restricted page tables are in use, and all other uses of
the per-domain area continue to use the "normal" VA.

> I would also consider giving the slot a more meaningful name, as
> PERDOMAIN2_VIRT_START makes it seem like a new per-domain scratch
> space, when it's just a different mapping of the existing physical
> memory.
> 
> Maybe PERDOMAIN_MIRROR_VIRT_START? Or PERDOMAIN_XLAT_VIRT_START?

XLAT would be too specific - while we use it for xlat only, it's
still all of the mappings that appear at the alternate addresses.
I did consider using MIRROR, but it got too long for my taste.
Now that I think about it maybe PERDOMAIN_ALT_VIRT_START would do?

Jan