[PATCH] x86: Check the lower byte of EAX of CPUID leaf 2 [BZ #30643]
Noah Goldstein
goldstein.w.n@gmail.com
Tue Aug 29 16:35:48 GMT 2023
On Mon, Aug 28, 2023 at 2:08 PM H.J. Lu via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> The old Intel software developer manual specified that the low byte of
> EAX of CPUID leaf 2 returned 1 which indicated the number of rounds of
> CPUDID leaf 2 was needed to retrieve the complete cache information. The
> newer Intel manual has been changed to that it should always return 1
> and be ignored. If the lower byte isn't 1, CPUID leaf 2 can't be used.
> In this case, we ignore CPUID leaf 2 and use CPUID leaf 4 instead. If
> CPUID leaf 4 doesn't contain the cache information, cache information
> isn't available at all. This addresses BZ #30643.
> ---
> sysdeps/x86/dl-cacheinfo.h | 31 +++++++++++++------------------
> 1 file changed, 13 insertions(+), 18 deletions(-)
>
> diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
> index 5ddb35c9d9..87486054f9 100644
> --- a/sysdeps/x86/dl-cacheinfo.h
> +++ b/sysdeps/x86/dl-cacheinfo.h
> @@ -187,7 +187,7 @@ intel_check_word (int name, unsigned int value, bool *has_level_2,
> ++round;
> }
> /* There is no other cache information anywhere else. */
> - break;
> + return -1;
> }
> else
> {
> @@ -257,28 +257,23 @@ handle_intel (int name, const struct cpu_features *cpu_features)
>
> /* OK, we can use the CPUID instruction to get all info about the
> caches. */
> - unsigned int cnt = 0;
> - unsigned int max = 1;
> long int result = 0;
> bool no_level_2_or_3 = false;
> bool has_level_2 = false;
> + unsigned int eax;
> + unsigned int ebx;
> + unsigned int ecx;
> + unsigned int edx;
> + __cpuid (2, eax, ebx, ecx, edx);
>
> - while (cnt++ < max)
> + /* The low byte of EAX of CPUID leaf 2 should always return 1 and it
> + should be ignored. If it isn't 1, use CPUID leaf 4 instead. */
> + if ((eax & 0xff) != 1)
> + return intel_check_word (name, 0xff, &has_level_2, &no_level_2_or_3,
> + cpu_features);
> + else
> {
> - unsigned int eax;
> - unsigned int ebx;
> - unsigned int ecx;
> - unsigned int edx;
> - __cpuid (2, eax, ebx, ecx, edx);
> -
> - /* The low byte of EAX in the first round contain the number of
> - rounds we have to make. At least one, the one we are already
> - doing. */
> - if (cnt == 1)
> - {
> - max = eax & 0xff;
> - eax &= 0xffffff00;
> - }
> + eax &= 0xffffff00;
>
> /* Process the individual registers' value. */
> result = intel_check_word (name, eax, &has_level_2,
> --
> 2.41.0
>
LGTM.
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