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KVM: VMX: Add a trampoline to fix VMREAD error handling

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Add a hand coded assembly trampoline to preserve volatile registers
across vmread_error(), and to handle the calling convention differences
between 64-bit and 32-bit due to asmlinkage on vmread_error().  Pass
@field and @fault on the stack when invoking the trampoline to avoid
clobbering volatile registers in the context of the inline assembly.

Calling vmread_error() directly from inline assembly is partially broken
on 64-bit, and completely broken on 32-bit.  On 64-bit, it will clobber
%rdi and %rsi (used to pass @field and @fault) and any volatile regs
written by vmread_error().  On 32-bit, asmlinkage means vmread_error()
expects the parameters to be passed on the stack, not via regs.

Opportunistically zero out the result in the trampoline to save a few
bytes of code for every VMREAD.  A happy side effect of the trampoline
is that the inline code footprint is reduced by three bytes on 64-bit
due to PUSH/POP being more efficent (in terms of opcode bytes) than MOV.

Fixes: 6e2020977e ("KVM: VMX: Add error handling to VMREAD helper")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200326160712.28803-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2020-03-26 09:07:12 -07:00 committed by Paolo Bonzini
parent 9c14ee21fc
commit 842f4be958
2 changed files with 79 additions and 7 deletions
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28
arch/x86/kvm/vmx/ops.h
View File

@ -12,7 +12,8 @@
#define __ex(x) __kvm_handle_fault_on_reboot(x)
asmlinkage void vmread_error(unsigned long field, bool fault);
__attribute__((regparm(0))) void vmread_error_trampoline(unsigned long field,
bool fault);
void vmwrite_error(unsigned long field, unsigned long value);
void vmclear_error(struct vmcs *vmcs, u64 phys_addr);
void vmptrld_error(struct vmcs *vmcs, u64 phys_addr);
@ -70,15 +71,28 @@ static __always_inline unsigned long __vmcs_readl(unsigned long field)
asm volatile("1: vmread %2, %1\n\t"
".byte 0x3e\n\t" /* branch taken hint */
"ja 3f\n\t"
"mov %2, %%" _ASM_ARG1 "\n\t"
"xor %%" _ASM_ARG2 ", %%" _ASM_ARG2 "\n\t"
"2: call vmread_error\n\t"
"xor %k1, %k1\n\t"
/*
* VMREAD failed. Push '0' for @fault, push the failing
* @field, and bounce through the trampoline to preserve
* volatile registers.
*/
"push $0\n\t"
"push %2\n\t"
"2:call vmread_error_trampoline\n\t"
/*
* Unwind the stack. Note, the trampoline zeros out the
* memory for @fault so that the result is '0' on error.
*/
"pop %2\n\t"
"pop %1\n\t"
"3:\n\t"
/* VMREAD faulted. As above, except push '1' for @fault. */
".pushsection .fixup, \"ax\"\n\t"
"4: mov %2, %%" _ASM_ARG1 "\n\t"
"mov $1, %%" _ASM_ARG2 "\n\t"
"4: push $1\n\t"
"push %2\n\t"
"jmp 2b\n\t"
".popsection\n\t"
_ASM_EXTABLE(1b, 4b)

58
arch/x86/kvm/vmx/vmenter.S
View File

@ -234,3 +234,61 @@ SYM_FUNC_START(__vmx_vcpu_run)
2: mov $1, %eax
jmp 1b
SYM_FUNC_END(__vmx_vcpu_run)
/**
* vmread_error_trampoline - Trampoline from inline asm to vmread_error()
* @field: VMCS field encoding that failed
* @fault: %true if the VMREAD faulted, %false if it failed
* Save and restore volatile registers across a call to vmread_error(). Note,
* all parameters are passed on the stack.
*/
SYM_FUNC_START(vmread_error_trampoline)
push %_ASM_BP
mov %_ASM_SP, %_ASM_BP
push %_ASM_AX
push %_ASM_CX
push %_ASM_DX
#ifdef CONFIG_X86_64
push %rdi
push %rsi
push %r8
push %r9
push %r10
push %r11
#endif
#ifdef CONFIG_X86_64
/* Load @field and @fault to arg1 and arg2 respectively. */
mov 3*WORD_SIZE(%rbp), %_ASM_ARG2
mov 2*WORD_SIZE(%rbp), %_ASM_ARG1
#else
/* Parameters are passed on the stack for 32-bit (see asmlinkage). */
push 3*WORD_SIZE(%ebp)
push 2*WORD_SIZE(%ebp)
#endif
call vmread_error
#ifndef CONFIG_X86_64
add $8, %esp
#endif
/* Zero out @fault, which will be popped into the result register. */
_ASM_MOV $0, 3*WORD_SIZE(%_ASM_BP)
#ifdef CONFIG_X86_64
pop %r11
pop %r10
pop %r9
pop %r8
pop %rsi
pop %rdi
#endif
pop %_ASM_DX
pop %_ASM_CX
pop %_ASM_AX
pop %_ASM_BP
ret
SYM_FUNC_END(vmread_error_trampoline)