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Subject[PATCH 00/16] KVM: x86: MMU page fault clean-up
Date
The original purpose of this series was to call thp_adjust() from
__direct_map() and FNAME(fetch) to eliminate a page refcounting quirk[*].
Before doing that, I wanted to clean up the large page handling so that
the map/fetch funtions weren't being passed multiple booleans that tracked
the same basic info. While trying to decipher all the the interactions,
I stumbled across a handful of fun things:

- 32-bit KVM w/ TDP is completely broken with respect to 64-bit GPAs due
to the page fault handlers and all related flows dropping bits 63:32
of the GPA. As a result, KVM inserts the wrong GPA and the guest hangs
because it generates EPT/NPT faults until it's killed.

- The TDP and non-paging page fault flows are identical except for
one-off constraints on guest page size.

- The !VALID_PAGE(root_hpa) checks in the page fault flows are bogus.
They were added a few years ago to "fix" a nVMX bug and are no longer
needed now that nVMX is in much better shape.

Patch 1 fixes the 32-bit KVM w/ TDP issue. More details below.

Patches 2-12 are 99% refactoring to merge TDP and non-paging page fault
handling, and to do the thp_adjust() move. These are basically nops from
a functional perspective. There are technically functional changes in a
few patches, but they are very superficial and in theory won't be
observable in normal usage.

Patches 13-16 add WARNs on the !VALID_PAGE(root_hpa) checks to make it
clear that root_hpa is expected to be valid when handling page faults,
e.g. for the longest time I thought KVM relied on the checks in map/fetch
to correctly handle kvm_mmu_zap_all().


32-bit KVM w/ TDP:

I marked this patch for stable because it's obviously a bug fix, but I'm
entirely not sure we want to backport the fix. Obviously no userspace VMM
is actually exposing 64-bit GPAs to its guests, i.e. odds are this won't
actually fix any real world use cases. And, the scope of the changes are
likely going to make backporting a pain. But, on the other hand, if it's
not backported then future bug fixes in related code are likely to
conflict, and it does fix the case where a buggy guest kernel accesses a
non-existent 64-bit GPA (crashes instead of hanging indefinitely).

I'm also not confident I found all the cases where KVM is truncating the
GPA. AFAIK, 32-bit Qemu simply doesn't support 64-bit GPAs. To confirm
the bug and verify the fix, I hacked KVM and the guest kernel to generate
64-bit GPAs when remapping MMIO, which covers a tiny fragment of KVM.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fa46fbed60013..49a59bcb32117 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5737,7 +5737,7 @@ static int emulator_read_write(struct x86_emulate_ctxt *ctxt,
vcpu->run->mmio.len = min(8u, vcpu->mmio_fragments[0].len);
vcpu->run->mmio.is_write = vcpu->mmio_is_write = ops->write;
vcpu->run->exit_reason = KVM_EXIT_MMIO;
- vcpu->run->mmio.phys_addr = gpa;
+ vcpu->run->mmio.phys_addr = gpa & 0xffffffffull;

return ops->read_write_exit_mmio(vcpu, gpa, val, bytes);
}

diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index b9c78f3bcd673..e22f254987bea 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -184,7 +184,10 @@ static void __iomem *__ioremap_caller(resource_size_t phys_addr,
if (kernel_map_sync_memtype(phys_addr, size, pcm))
goto err_free_area;

- if (ioremap_page_range(vaddr, vaddr + size, phys_addr, prot))
+ BUG_ON(!boot_cpu_data.x86_phys_bits);
+
+ if (ioremap_page_range(vaddr, vaddr + size,
+ phys_addr | BIT_ULL(boot_cpu_data.x86_phys_bits - 1), prot))
goto err_free_area;

ret_addr = (void __iomem *) (vaddr + offset);
[*] https://lkml.kernel.org/r/20191126174603.GB22233@linux.intel.com

Sean Christopherson (16):
KVM: x86: Use gpa_t for cr2/gpa to fix TDP support on 32-bit KVM
KVM: x86/mmu: Move definition of make_mmu_pages_available() up
KVM: x86/mmu: Fold nonpaging_map() into nonpaging_page_fault()
KVM: x86/mmu: Move nonpaging_page_fault() below try_async_pf()
KVM: x86/mmu: Refactor handling of cache consistency with TDP
KVM: x86/mmu: Refactor the per-slot level calculation in
mapping_level()
KVM: x86/mmu: Refactor handling of forced 4k pages in page faults
KVM: x86/mmu: Incorporate guest's page level into max level for shadow
MMU
KVM: x86/mmu: Persist gfn_lpage_is_disallowed() to max_level
KVM: x86/mmu: Rename lpage_disallowed to account_disallowed_nx_lpage
KVM: x86/mmu: Consolidate tdp_page_fault() and nonpaging_page_fault()
KVM: x86/mmu: Move transparent_hugepage_adjust() above __direct_map()
KVM: x86/mmu: Move calls to thp_adjust() down a level
KVM: x86/mmu: Move root_hpa validity checks to top of page fault
handler
KVM: x86/mmu: WARN on an invalid root_hpa
KVM: x86/mmu: WARN if root_hpa is invalid when handling a page fault

arch/x86/include/asm/kvm_host.h | 8 +-
arch/x86/kvm/mmu/mmu.c | 438 ++++++++++++++------------------
arch/x86/kvm/mmu/paging_tmpl.h | 58 +++--
arch/x86/kvm/mmutrace.h | 12 +-
arch/x86/kvm/x86.c | 40 ++-
arch/x86/kvm/x86.h | 2 +-
include/linux/kvm_host.h | 6 +-
virt/kvm/async_pf.c | 10 +-
8 files changed, 259 insertions(+), 315 deletions(-)
--
2.24.0

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