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KVM: x86/mmu: rename and clarify BYTE_MASK

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The BYTE_MASK macro is the central point of the black magic
in update_permission_bitmask().  Rename it to something
that relates to how it is used, and add a comment explaining
how it works.

Using shifts instead of powers of two was actually suggested by
David Hildenbrand back in 2017 for clarity[1] but I evidently
forgot his suggestion when applying to kvm.git.

[1] https://lore.kernel.org/kvm/e4b5df86-31ae-2f4e-0666-393753e256df@redhat.com/

Tested-by: David Riley <d.riley@proxmox.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2026-04-08 11:41:57 -04:00
parent 0c768fa0ae
commit 296b29597d
1 changed files with 43 additions and 20 deletions
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63
arch/x86/kvm/mmu/mmu.c
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@ -5529,31 +5529,55 @@ reset_ept_shadow_zero_bits_mask(struct kvm_mmu *context, bool execonly)
max_huge_page_level);
}
#define BYTE_MASK(access) \
((1 & (access) ? 2 : 0) | \
(2 & (access) ? 4 : 0) | \
(3 & (access) ? 8 : 0) | \
(4 & (access) ? 16 : 0) | \
(5 & (access) ? 32 : 0) | \
(6 & (access) ? 64 : 0) | \
(7 & (access) ? 128 : 0))
/*
* Build a mask with all combinations of PTE access rights that
* include the given access bit. The mask can be queried with
* "mask & (1 << access)", where access is a combination of
* ACC_* bits.
*
* By mixing and matching multiple masks returned by ACC_BITS_MASK,
* update_permission_bitmask() builds what is effectively a
* two-dimensional array of bools. The second dimension is
* provided by individual bits of permissions[pfec >> 1], and
* logical &, | and ~ operations operate on all the 8 possible
* combinations of ACC_* bits.
*/
#define ACC_BITS_MASK(access) \
((1 & (access) ? 1 << 1 : 0) | \
(2 & (access) ? 1 << 2 : 0) | \
(3 & (access) ? 1 << 3 : 0) | \
(4 & (access) ? 1 << 4 : 0) | \
(5 & (access) ? 1 << 5 : 0) | \
(6 & (access) ? 1 << 6 : 0) | \
(7 & (access) ? 1 << 7 : 0))
static void update_permission_bitmask(struct kvm_mmu *mmu, bool ept)
{
unsigned byte;
unsigned index;
const u8 x = BYTE_MASK(ACC_EXEC_MASK);
const u8 w = BYTE_MASK(ACC_WRITE_MASK);
const u8 u = BYTE_MASK(ACC_USER_MASK);
const u8 x = ACC_BITS_MASK(ACC_EXEC_MASK);
const u8 w = ACC_BITS_MASK(ACC_WRITE_MASK);
const u8 u = ACC_BITS_MASK(ACC_USER_MASK);
bool cr4_smep = is_cr4_smep(mmu);
bool cr4_smap = is_cr4_smap(mmu);
bool cr0_wp = is_cr0_wp(mmu);
bool efer_nx = is_efer_nx(mmu);
for (byte = 0; byte < ARRAY_SIZE(mmu->permissions); ++byte) {
unsigned pfec = byte << 1;
/*
* In hardware, page fault error codes are generated (as the name
* suggests) on any kind of page fault. permission_fault() and
* paging_tmpl.h already use the same bits after a successful page
* table walk, to indicate the kind of access being performed.
*
* However, PFERR_PRESENT_MASK and PFERR_RSVD_MASK are never set here,
* exactly because the page walk is successful. PFERR_PRESENT_MASK is
* removed by the shift, while PFERR_RSVD_MASK is repurposed in
* permission_fault() to indicate accesses that are *not* subject to
* SMAP restrictions.
*/
for (index = 0; index < ARRAY_SIZE(mmu->permissions); ++index) {
unsigned pfec = index << 1;
/*
* Each "*f" variable has a 1 bit for each UWX value
@ -5598,16 +5622,15 @@ static void update_permission_bitmask(struct kvm_mmu *mmu, bool ept)
* - The access is supervisor mode
* - If implicit supervisor access or X86_EFLAGS_AC is clear
*
* Here, we cover the first four conditions.
* The fifth is computed dynamically in permission_fault();
* PFERR_RSVD_MASK bit will be set in PFEC if the access is
* *not* subject to SMAP restrictions.
* Here, we cover the first four conditions. The fifth
* is computed dynamically in permission_fault() and
* communicated by setting PFERR_RSVD_MASK.
*/
if (cr4_smap)
smapf = (pfec & (PFERR_RSVD_MASK|PFERR_FETCH_MASK)) ? 0 : kf;
}
mmu->permissions[byte] = ff | uf | wf | smepf | smapf;
mmu->permissions[index] = ff | uf | wf | smepf | smapf;
}
}