On Fri, Jul 15, 2022 at 07:49:27AM -0400, Paolo Bonzini wrote:
> Instead of doing complicated calculations to find the size of the subroutines
> (which are even more complicated because they need to be stringified into
> an asm statement), just hardcode to 16.
>
> It is less dense for a few combinations of IBT/SLS/retbleed, but it has
> the advantage of being really simple.
>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> arch/x86/kvm/emulate.c | 17 +++++++----------
> 1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 0a15b0fec6d9..f8382abe22ff 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -189,13 +189,6 @@
> #define X8(x...) X4(x), X4(x)
> #define X16(x...) X8(x), X8(x)
>
> -#define NR_FASTOP (ilog2(sizeof(ulong)) + 1)
> -#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETHUNK)) + \
> - IS_ENABLED(CONFIG_SLS))
> -#define FASTOP_LENGTH (ENDBR_INSN_SIZE + 7 + RET_LENGTH)
> -#define FASTOP_SIZE (8 << ((FASTOP_LENGTH > 8) & 1) << ((FASTOP_LENGTH > 16) & 1))
> -static_assert(FASTOP_LENGTH <= FASTOP_SIZE);
> -
> struct opcode {
> u64 flags;
> u8 intercept;
> @@ -310,9 +303,15 @@ static void invalidate_registers(struct x86_emulate_ctxt *ctxt)
> * Moreover, they are all exactly FASTOP_SIZE bytes long, so functions for
> * different operand sizes can be reached by calculation, rather than a jump
> * table (which would be bigger than the code).
> + *
> + * The 16 byte alignment, considering 5 bytes for the RET thunk, 3 for ENDBR
> + * and 1 for the straight line speculation INT3, leaves 7 bytes for the
> + * body of the function. Currently none is larger than 4.
The ENDBR is 4 bytes long, which leaves only 6 bytes left. Which is why the
calculation being replaced may end up with a FASTOP_SIZE of 32.
Can you fix up these numbers when applying?
Cascardo.
> */
> static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
>
> +#define FASTOP_SIZE 16
> +
> #define __FOP_FUNC(name) \
> ".align " __stringify(FASTOP_SIZE) " \n\t" \
> ".type " name ", @function \n\t" \
> @@ -446,9 +445,7 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
> * RET | JMP __x86_return_thunk [1,5 bytes; CONFIG_RETHUNK]
> * INT3 [1 byte; CONFIG_SLS]
> */
> -#define SETCC_LENGTH (ENDBR_INSN_SIZE + 3 + RET_LENGTH)
> -#define SETCC_ALIGN (4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1))
> -static_assert(SETCC_LENGTH <= SETCC_ALIGN);
> +#define SETCC_ALIGN 16
>
> #define FOP_SETCC(op) \
> ".align " __stringify(SETCC_ALIGN) " \n\t" \
> --
> 2.31.1
>
