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KVM: x86: Use 16-bit fields to track dirty/valid emulator GPRs

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Use a u16 instead of a u32 to track the dirty/valid status of GPRs in the
emulator.  Unlike struct kvm_vcpu_arch, x86_emulate_ctxt tracks only the
"true" GPRs, i.e. doesn't include RIP in its array, and so only needs to
track 16 registers.

Note, maxing out at 16 GPRs is a fundamental property of x86-64 and will
not change barring a massive architecture update.  Legacy x86 ModRM and
SIB encodings use 3 bits for GPRs, i.e. support 8 registers.  x86-64 uses
a single bit in the REX prefix for each possible reference type to double
the number of supported GPRs to 16 registers (4 bits).

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220526210817.3428868-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson
2022-05-26 21:08:13 +00:00
committed by Paolo Bonzini
parent a5ba67b42f
commit 0cbc60d44c
2 changed files with 5 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

3
arch/x86/kvm/emulate.c
View File

@ -262,6 +262,9 @@ static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr)
if (WARN_ON_ONCE(nr >= NR_EMULATOR_GPRS)) if (WARN_ON_ONCE(nr >= NR_EMULATOR_GPRS))
nr &= NR_EMULATOR_GPRS - 1; nr &= NR_EMULATOR_GPRS - 1;
BUILD_BUG_ON(sizeof(ctxt->regs_dirty) * BITS_PER_BYTE < NR_EMULATOR_GPRS);
BUILD_BUG_ON(sizeof(ctxt->regs_valid) * BITS_PER_BYTE < NR_EMULATOR_GPRS);
ctxt->regs_valid |= 1 << nr; ctxt->regs_valid |= 1 << nr;
ctxt->regs_dirty |= 1 << nr; ctxt->regs_dirty |= 1 << nr;
return &ctxt->_regs[nr]; return &ctxt->_regs[nr];

4
arch/x86/kvm/kvm_emulate.h
View File

@ -356,9 +356,9 @@ struct x86_emulate_ctxt {
u8 lock_prefix; u8 lock_prefix;
u8 rep_prefix; u8 rep_prefix;
/* bitmaps of registers in _regs[] that can be read */ /* bitmaps of registers in _regs[] that can be read */
u32 regs_valid; u16 regs_valid;
/* bitmaps of registers in _regs[] that have been written */ /* bitmaps of registers in _regs[] that have been written */
u32 regs_dirty; u16 regs_dirty;
/* modrm */ /* modrm */
u8 modrm; u8 modrm;
u8 modrm_mod; u8 modrm_mod;