On 22.09.2021 12:28, Julien Grall wrote: > Hi, > > On 22/09/2021 14:42, Jan Beulich wrote: >> On 22.09.2021 11:26, Roger Pau Monné wrote: >>> On Tue, Sep 21, 2021 at 12:12:05PM +0200, Jan Beulich wrote: >>>> On 21.09.2021 10:32, Roger Pau Monné wrote: >>>>> On Mon, Sep 20, 2021 at 05:27:17PM +0200, Jan Beulich wrote: >>>>>> On 20.09.2021 12:20, Roger Pau Monné wrote: >>>>>>> On Mon, Sep 13, 2021 at 08:41:47AM +0200, Jan Beulich wrote: >>>>>>>> --- a/xen/include/asm-arm/grant_table.h >>>>>>>> +++ b/xen/include/asm-arm/grant_table.h >>>>>>>> + if ( gfn_eq(ogfn, INVALID_GFN) || gfn_eq(ogfn, gfn) || >>>>>>>> \ >>>>>>> >>>>>>> I'm slightly confused by this checks, don't you need to check for >>>>>>> gfn_eq(gfn, INVALID_GFN) (not ogfn) in order to call >>>>>>> guest_physmap_remove_page? >>>>>> >>>>>> Why? It's ogfn which gets passed to the function. And it indeed is the >>>>>> prior GFN's mapping that we want to remove here. >>>>>> >>>>>>> Or assuming that ogfn is not invalid can be used to imply a removal? >>>>>> >>>>>> That implication can be (and on x86 is) used for the incoming argument, >>>>>> i.e. "gfn". I don't think "ogfn" can serve this purpose. >>>>> >>>>> I guess I'm confused due to the ogfn checks done on the Arm side that >>>>> are not performed on x86. So on Arm you always need to explicitly >>>>> unhook the previous GFN before attempting to setup a new mapping, >>>>> while on x86 you only need to do this when it's a removal in order to >>>>> clear the entry? >>>> >>>> The difference isn't with guest_physmap_add_entry() (both x86 and >>>> Arm only insert a new mapping there), but with >>>> xenmem_add_to_physmap_one(): Arm's variant doesn't care about prior >>>> mappings. And gnttab_map_frame() gets called only from there. This >>>> is effectively what the first paragraph of the description is about. >>> >>> OK, sorry, it wasn't clear to me from the description. Could you >>> explicitly mention in the description that the removal is moved into >>> gnttab_set_frame_gfn on Arm in order to cope with the fact that >>> xenmem_add_to_physmap_one doesn't perform it. >> >> Well, it's not really "in order to cope" - that's true for the placement >> prior to this change as well, so not a justification for the change. >> Nevertheless I've tried to make this more clear by changing the 1st >> paragraph to: >> >> "Without holding appropriate locks, attempting to remove a prior mapping >> of the underlying page is pointless, as the same (or another) mapping >> could be re-established by a parallel request on another vCPU. Move the >> code to Arm's gnttab_set_frame_gfn(); it cannot be dropped there since >> xenmem_add_to_physmap_one() doesn't call it either (unlike on x86). Of >> course this new placement doesn't improve things in any way as far as >> the security of grant status frame mappings goes (see XSA-379). Proper >> locking would be needed here to allow status frames to be mapped >> securely." >> >>> TBH I think it would be in our best interest to try to make >>> xenmem_add_to_physmap_one behave as close as possible between arches. >>> This discrepancy between x86 and Arm regarding page removal is just >>> going to bring more trouble in the long term, and hiding the >>> differences inside gnttab_set_frame_gfn just makes it even more >>> obscure. >> >> Stefano, Julien? > > This would be ideal as I don't particular like the approach taken in > this patch. But AFAICT, this would require us to implement an M2P. Or is > there another way to do it?
For the purpose of just XENMAPSPACE_grant_table the "restricted" M2P (to just grant table pages), as introduced by an on-list patch, would do afaict. That wouldn't remove all the differences between the two implementations, but at least the one affecting the difference in how gnttab_map_frame() needs to behave. Jan
