Re: [PATCH v2 01/13] KVM: arm64: Donate MMIO to the hypervisor
From: Fuad Tabba <fuad.tabba@linux.dev>
Date: 2026-09-09 15:39:47
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kvmarm, lkml
Hi Mostaf, Seb, On Fri, 7 Aug 2026 at 17:43, Sebastian Ene [off-list ref] wrote:
From: Mostafa Saleh <smostafa@google.com>
...
+int __pkvm_host_donate_hyp_mmio(phys_addr_t addr, size_t size)
+{...
quoted hunk ↗ jump to hunk
+ /* + * We set HYP as the owner of the MMIO pages in the host stage-2, for: + * - host aborts: host_stage2_adjust_range() would fail for invalid non zero PTEs. + * - recycle under memory pressure: host_stage2_unmap_dev_all() would call + * kvm_pgtable_stage2_unmap() which will not clear non zero invalid ptes (counted). + * - other MMIO donation: Would fail as we check that the PTE is valid or empty. + */ + ret = host_stage2_try(kvm_pgtable_stage2_annotate, &host_mmu.pgt, + addr, size, &host_s2_pool, + KVM_HOST_INVALID_PTE_TYPE_DONATION, + FIELD_PREP(KVM_HOST_DONATION_PTE_OWNER_MASK, PKVM_ID_HYP));
Before this patch the hyp linear map held nothing but memory, and this adds the first device mapping to it. That places it outside `fix_host_ownership()`, whose scope is the memblock list, so nothing at init verifies the ownership. It's correct as written: the donation writes the host stage-2 annotation itself. This has already gone wrong once with the hyp stacks. The fix [1] adds `pkvm_check_host_ownership()` over the private range, which fails init on a leaf that isn't hyp-owned. I'd move this mapping there rather than leave it outside any ownership walk. ...
+int __pkvm_hyp_donate_host_mmio(phys_addr_t addr, size_t size)
+{...
quoted hunk ↗ jump to hunk
+ virt = __hyp_va(addr + offset); + if (kvm_pgtable_hyp_unmap(&pkvm_pgtable, (u64)virt, PAGE_SIZE) != PAGE_SIZE) + goto err_with_unmap;
Sashiko is right, even though this is benign for now. `ret` is still 0 from the `kvm_pgtable_get_leaf()` above, so a short unmap runs the rollback and returns success. Could it set an error before the goto? ...
quoted hunk ↗ jump to hunk
@@ -1161,13 +1161,12 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t *childp = NULL; bool need_flush = false; - if (!kvm_pte_valid(ctx->old)) { - if (stage2_pte_is_counted(ctx->old)) { - kvm_clear_pte(ctx->ptep); - mm_ops->put_page(ctx->ptep); - } + /* + * That also ignores stage2_pte_is_counted() instead of clearing + * the PTE as the MMIO can be owned by the hypervisor. + */ + if (!kvm_pte_valid(ctx->old)) return 0; - }
This changes `kvm_pgtable_stage2_unmap()` for every caller, for a reason specific to `host_stage2_unmap_dev_all()`. The others are `__unmap_stage2_range()` and the two guest unmaps in `mem_protect.c`; as far as I can tell none is affected today, but `stage2_pte_is_counted()`'s comment still describes the old behaviour. What's changing isn't what the walk does, it's what counts as a counted entry, and that isn't the same question for the host stage-2 as for a guest's. Could that be stated at the call site instead? I couldn't work out what "That" refers to in the new comment. Cheers, /fuad [1] https://lore.kernel.org/all/20260908110713.1540304-1-fuad.tabba@linux.dev/ (local)
if (kvm_pte_table(ctx->old, ctx->level)) {
childp = kvm_pte_follow(ctx->old, mm_ops);
--
2.55.0.654.g21b8a5bc05-goog