Re: [PATCH] KVM: emulate: do not adjust size of fastop and setcc subroutines

[Thadeu Lima de Souza Cascardo <cascardo@canonical.com>]

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@linux-foundation.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  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
>