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Current issue is > > > > > the changes with the handling of the shared information page appear to > > > > > have broken things for me. > > > > > > > > > > With a pre-4.17 build of Xen/ARM things worked fine. Yet with a build > > > > > of the 4.17 release, mapping the shared information page doesn't work. > > > > > > > > This is due to 71320946d5edf AFAICT. > > > > > > Yes. While the -EBUSY line may be the one triggering, I'm unsure why. > > > This seems a fairly reasonable change, so I had no intention of asking > > > for a revert (which likely would have been rejected). There is also a > > > real possibility the -EBUSY comes from elsewhere. Could also be > > > 71320946d5edf caused a bug elsewhere to be exposed. > > > > A good way to know would be to attempt to revert 71320946d5edf and see > > if that fixes your issue. > > > > Alternatively you can try (or similar): > > > > diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c > > index 6ccffeaea57d..105ef3faecfd 100644 > > --- a/xen/arch/arm/mm.c > > +++ b/xen/arch/arm/mm.c > > @@ -1424,6 +1424,8 @@ int xenmem_add_to_physmap_one( > > page_set_xenheap_gfn(mfn_to_page(mfn), gfn); > > } > > else > > + { > > + printk("%u already mapped\n", space); > > /* > > * Mandate the caller to first unmap the page before mapping it > > * again. This is to prevent Xen creating an unwanted hole in > > @@ -1432,6 +1434,7 @@ int xenmem_add_to_physmap_one( > > * to unmap it afterwards. > > */ > > rc = -EBUSY; > > + } > > p2m_write_unlock(p2m); > > } > > > > > > > I'm using Tianocore as the first stage loader. This continues to work > > > > > fine. The build is using tag "edk2-stable202211", commit fff6d81270. > > > > > While Tianocore does map the shared information page, my reading of their > > > > > source is that it properly unmaps the page and therefore shouldn't cause > > > > > trouble. > > > > > > > > > > Notes on the actual call is gpfn was 0x0000000000040072. This is outside > > > > > the recommended address range, but my understanding is this is supposed > > > > > to be okay. > > > > > > > > > > The return code is -16, which is EBUSY. > > > > > > > > > > Ideas? > > > > > > > > I think the issue is that you are mapping the shared info page over a > > > > guest RAM page, and in order to do that you would fist need to create > > > > a hole and then map the shared info page. IOW: the issue is not with > > > > edk2 not having unmapped the page, but with FreeBSD trying to map the > > > > shared_info over a RAM page instead of a hole in the p2m. x86 > > > > behavior is different here, and does allow mapping the shared_info > > > > page over a RAM gfn (by first removing the backing RAM page on the > > > > gfn). > > > > > > An interesting thought. I thought I'd tried this, but since I didn't see > > > such in my experiments list. What I had tried was removing all the pages > > > in the suggested mapping range. Yet this failed. > > > > Yeah, I went too fast and didn't read the code correctly, it is not > > checking that the provided gfn is already populated, but whether the > > mfn intended to be mapped is already mapped at a different location. > > > > > Since this seemed reasonable, I've now tried and found it fails. The > > > XENMEM_remove_from_physmap call returns 0. > > > > XENMEM_remove_from_physmap returning 0 is fine, but it seems to me > > like edk2 hasn't unmapped the shared_info page. The OS has no idea > > at which position the shared_info page is currently mapped, and hence > > can't do anything to attempt to unmap it in order to cover up for > > buggy firmware. > > > > edk2 should be the entity to issue the XENMEM_remove_from_physmap > > against the gfn where it has the shared_info page mapped. Likely > > needs to be done as part of ExitBootServices() method. > > > > FWIW, 71320946d5edf is an ABI change, and as desirable as such > > behavior might be, a new hypercall should have introduced that had the > > behavior that the change intended to retrofit into > > XENMEM_add_to_physmap. > I can see how you think this is an ABI change but the previous behavior was > incorrect. Before this patch, on Arm, we would allow the shared page to be > mapped twice. As we don't know where the firmware had mapped it this could > result to random corruption. > > Now, we could surely decide to remove the page as x86 did. But this could > leave a hole in the RAM. As the OS would not know where the hole is, this > could lead to page fault randomly during runtime. I would say it's the job of the firmware to notify the OS where the hole is, by modifying the memory map handled to the OS. Or else mapping the shared_info page in an unpopulated p2m hole. When using UEFI there's RAM that will always be in-use by the firmware, as runtime services cannot be shut down, and hence the firmware must already have a way to remove/reserve such region(s) on the memory map. > Neither of the two behaviors help the users. In fact, I think they only make > the experience worse because you don't know when the issue will happen. > > AFAICT, there is no way for an HVM guestto know which GFN was inuse. So in > all the cases, I can't think of a way for the OS to workaround properly > buggy firmware. Therefore, returning -EBUSY is the safest we can do for our > users and I don't view it as a ABI change (someone rely on the previous > behavior is bound to failure). I fully agree the current behavior might not be the best one, but I do consider this part of the ABI, specially as booting guests using edk2 has now stopped working after this change. Introducing a different hypercall, or even using XENMAPSPACE_shared_info with idx = 1 to signal the usage of the new behavior should be used instead. This would also allow unifying the behavior between x86 and Arm. Thanks, Roger.