On 22.05.2025 17:00, Andrew Cooper wrote:
> The {k8,p6}_nops[] arrays are both 80-byte structures indexing 45-byte
> structures. Furthermore, perhaps unusually for C, the source layout is an
> obvious hint about the trangular nature of the structure.
>
> Therefore, we can replace the pointer chase with some simple arithmatic.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <[email protected]>
> ---
> CC: Jan Beulich <[email protected]>
> CC: Roger Pau Monné <[email protected]>
>
> The implemenation of get_ideal_nops() changes from:
>
> mov 0x19bc41(%rip),%rax # <ideal_nops>
> mov %edi,%edi
> mov (%rax,%rdi,8),%rax
> jmp <__x86_return_thunk>
>
> to:
>
> lea -0x1(%rdi),%eax
> imul %edi,%eax
> shr %eax
> add 0x67fc1(%rip),%rax # <ideal_nops>
> jmp <__x86_return_thunk>
>
> The imul has a latency of 3 cycles on all CPUs back to the K8 and Nehalem.
> It's better than an extra deference on all CPUs, even the older ones.
While this is all good, what we're losing is ...
> --- a/xen/arch/x86/alternative.c
> +++ b/xen/arch/x86/alternative.c
> @@ -20,7 +20,7 @@
> #define MAX_PATCH_LEN (255-1)
>
> #ifdef K8_NOP1
> -static const unsigned char k8nops[] init_or_livepatch_const = {
> +static const unsigned char k8_nops[] init_or_livepatch_const = {
> K8_NOP1,
> K8_NOP2,
> K8_NOP3,
> @@ -31,22 +31,10 @@ static const unsigned char k8nops[]
> init_or_livepatch_const = {
> K8_NOP8,
> K8_NOP9,
> };
> -static const unsigned char * const k8_nops[ASM_NOP_MAX+1]
> init_or_livepatch_constrel = {
... the (at least visual) connection to ASM_NOP_MAX. Could I talk you into
adding build time array-size checks for both arrays, to restore the
connection?
> - NULL,
> - k8nops,
> - k8nops + 1,
> - k8nops + 1 + 2,
> - k8nops + 1 + 2 + 3,
> - k8nops + 1 + 2 + 3 + 4,
> - k8nops + 1 + 2 + 3 + 4 + 5,
> - k8nops + 1 + 2 + 3 + 4 + 5 + 6,
> - k8nops + 1 + 2 + 3 + 4 + 5 + 6 + 7,
> - k8nops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8,
> -};
> #endif
>
> #ifdef P6_NOP1
> -static const unsigned char p6nops[] init_or_livepatch_const = {
> +static const unsigned char p6_nops[] init_or_livepatch_const = {
> P6_NOP1,
> P6_NOP2,
> P6_NOP3,
> @@ -57,21 +45,9 @@ static const unsigned char p6nops[]
> init_or_livepatch_const = {
> P6_NOP8,
> P6_NOP9,
> };
> -static const unsigned char * const p6_nops[ASM_NOP_MAX+1]
> init_or_livepatch_constrel = {
> - NULL,
> - p6nops,
> - p6nops + 1,
> - p6nops + 1 + 2,
> - p6nops + 1 + 2 + 3,
> - p6nops + 1 + 2 + 3 + 4,
> - p6nops + 1 + 2 + 3 + 4 + 5,
> - p6nops + 1 + 2 + 3 + 4 + 5 + 6,
> - p6nops + 1 + 2 + 3 + 4 + 5 + 6 + 7,
> - p6nops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8,
> -};
> #endif
>
> -static const unsigned char * const *ideal_nops init_or_livepatch_data =
> p6_nops;
> +static const unsigned char *ideal_nops init_or_livepatch_data = p6_nops;
>
> #ifdef HAVE_AS_NOPS_DIRECTIVE
>
> @@ -86,9 +62,17 @@ static bool init_or_livepatch_read_mostly
> toolchain_nops_are_ideal;
> # define toolchain_nops_are_ideal false
> #endif
>
> +/*
> + * Both k8_nops[] and p6_nops[] are flattened triangular data structures,
> + * making the offsets easy to calculate.
> + *
> + * To get the start of NOP $N, we want to calculate T($N - 1)
> + */
> static const unsigned char *get_ideal_nops(unsigned int noplen)
> {
> - return ideal_nops[noplen];
> + unsigned int offset = ((noplen - 1) * noplen) / 2;
> +
> + return &ideal_nops[offset];
> }
>
> static void __init arch_init_ideal_nops(void)
