On Mon, Feb 22, 2021 at 03:20:24PM +0100, Jan Beulich wrote: > 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.
Oh, OK, thanks for the clarification. AFAICT the PERDOMAIN2_VIRT_START slot won't get populated on the restricted page tables, and hence will always trigger a page fault if access is attempted with those tables loaded. > > 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. Well, given that such mappings won't be available when running 64bit PV guests I still think it's unlikely to be used for anything that's not XLAT specific, as it won't work for 64bit PV guests otherwise. > 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? Indeed, I would prefer that rather than PERDOMAIN2_VIRT_START if you still consider XLAT to be too specific. Thanks, Roger.
