Re: [PATCH for v10.1] i386/cpu: Move addressable ID encoding out of compat property in CPUID[0x1]

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 4/8/25 07:35, Zhao Liu wrote:
> Currently, the addressable ID encoding for CPUID[0x1].EBX[bits 16-23]
> (Maximum number of addressable IDs for logical processors in this
> physical package) is covered by vendor_cpuid_only_v2 compat property.
> The previous consideration was to avoid breaking migration and this
> compat property makes it unfriendly to backport the commit f985a1195ba2
> ("i386/cpu: Fix number of addressable IDs field for CPUID.01H.EBX
> [23:16]").
> 
> However, NetBSD booting is broken since the commit 88dd4ca06c83
> ("i386/cpu: Use APIC ID info to encode cache topo in CPUID[4]"),
> because NetBSD calculates smt information via `lp_max` / `core_max` for
> legacy Intel CPUs which doesn't support 0xb leaf, where `lp_max` is from
> CPUID[0x1].EBX.bits[16-23] and `core_max` is from CPUID[0x4].0x0.bits[26
> -31].
> 
> The commit 88dd4ca0 changed the encoding rule of `core_max` but didn't
> update `lp_max`, so that NetBSD would get the wrong smt information,
> which leads to the module loading failure.
> 
> Luckily, the commit f985a1195ba2 ("i386/cpu: Fix number of addressable
> IDs field for CPUID.01H.EBX[23:16]") updated the encoding rule for
> `lp_max` and accidentally fixed the NetBSD issue too. This also shows
> that using CPUID[0x1] and CPUID[0x4].0x0 to calculate HT/SMT information
> is a common practice to detect CPU topology on legacy Intel CPUs.
> 
> Therefore, it's necessary to backport the commit f985a1195ba2 to
> previous stable QEMU to help address the similar issues as well. Then
> the compat property is not needed any more since all stable QEMUs will
> follow the same encoding way.
> 
> So, in CPUID[0x1], move addressable ID encoding out of compat property.
> 
> Reported-by: Michael Tokarev <mjt@tls.msk.ru>
> Inspired-by: Chuang Xu <xuchuangxclwt@bytedance.com>
> Fixes: commit f985a1195ba2 ("i386/cpu: Fix number of addressable IDs field for CPUID.01H.EBX[23:16]")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3061
> Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> ---
>   target/i386/cpu.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)

Patch queued, thanks!