Re: [PATCH v2] target/i386: Fix CPUID encoding of Fn8000001E_ECX

[Moger, Babu]

Any feedback or concerns with this patch? Otherwise can this be merged?
Thanks
Babu

On 1/2/24 17:17, Babu Moger wrote:
> Observed the following failure while booting the SEV-SNP guest and the
> guest fails to boot with the smp parameters:
> "-smp 192,sockets=1,dies=12,cores=8,threads=2".
> 
> qemu-system-x86_64: sev_snp_launch_update: SNP_LAUNCH_UPDATE ret=-5 
> fw_error=22 'Invalid parameter'
> qemu-system-x86_64: SEV-SNP: CPUID validation failed for function 0x8000001e, 
> index: 0x0.
> provided: eax:0x00000000, ebx: 0x00000100, ecx: 0x00000b00, edx: 0x00000000
> expected: eax:0x00000000, ebx: 0x00000100, ecx: 0x00000300, edx: 0x00000000
> qemu-system-x86_64: SEV-SNP: failed update CPUID page
> 
> Reason for the failure is due to overflowing of bits used for "Node per
> processor" in CPUID Fn8000001E_ECX. This field's width is 3 bits wide and
> can hold maximum value 0x7. With dies=12 (0xB), it overflows and spills
> over into the reserved bits. In the case of SEV-SNP, this causes CPUID
> enforcement failure and guest fails to boot.
> 
> The PPR documentation for CPUID_Fn8000001E_ECX [Node Identifiers]
> =================================================================
> Bits    Description
> 31:11   Reserved.
> 
> 10:8    NodesPerProcessor: Node per processor. Read-only.
>         ValidValues:
>         Value   Description
>         0h      1 node per processor.
>         7h-1h   Reserved.
> 
> 7:0     NodeId: Node ID. Read-only. Reset: Fixed,XXh.
> =================================================================
> 
> As in the spec, the valid value for "node per processor" is 0 and rest
> are reserved.
> 
> Looking back at the history of decoding of CPUID_Fn8000001E_ECX, noticed
> that there were cases where "node per processor" can be more than 1. It
> is valid only for pre-F17h (pre-EPYC) architectures. For EPYC or later
> CPUs, the linux kernel does not use this information to build the L3
> topology.
> 
> Also noted that the CPUID Function 0x8000001E_ECX is available only when
> TOPOEXT feature is enabled. This feature is enabled only for EPYC(F17h)
> or later processors. So, previous generation of processors do not not
> enumerate 0x8000001E_ECX leaf.
> 
> There could be some corner cases where the older guests could enable the
> TOPOEXT feature by running with -cpu host, in which case legacy guests
> might notice the topology change. To address those cases introduced a
> new CPU property "legacy-multi-node". It will be true for older machine
> types to maintain compatibility. By default, it will be false, so new
> decoding will be used going forward.
> 
> The documentation is taken from Preliminary Processor Programming
> Reference (PPR) for AMD Family 19h Model 11h, Revision B1 Processors 55901
> Rev 0.25 - Oct 6, 2022.
> 
> Cc: qemu-stable@nongnu.org
> Fixes: 31ada106d891 ("Simplify CPUID_8000_001E for AMD")
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
> Signed-off-by: Babu Moger <babu.moger@amd.com>
> Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
> ---
> v2: Rebased to the latest tree.
>     Updated the pc_compat_8_2 for the new flag.
>     Added the comment for new property legacy_multi_node.
>     Added Reviwed-by from Zhao.
> ---
>  hw/i386/pc.c      |  4 +++-
>  target/i386/cpu.c | 18 ++++++++++--------
>  target/i386/cpu.h |  6 ++++++
>  3 files changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 496498df3a..a504e05e62 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -78,7 +78,9 @@
>      { "qemu64-" TYPE_X86_CPU, "model-id", "QEMU Virtual CPU version " v, },\
>      { "athlon-" TYPE_X86_CPU, "model-id", "QEMU Virtual CPU version " v, },
>  
> -GlobalProperty pc_compat_8_2[] = {};
> +GlobalProperty pc_compat_8_2[] = {
> +    { TYPE_X86_CPU, "legacy-multi-node", "on" },
> +};
>  const size_t pc_compat_8_2_len = G_N_ELEMENTS(pc_compat_8_2);
>  
>  GlobalProperty pc_compat_8_1[] = {};
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 95d5f16cd5..2cc84e8500 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -398,12 +398,9 @@ static void encode_topo_cpuid8000001e(X86CPU *cpu, 
> X86CPUTopoInfo *topo_info,
>       * 31:11 Reserved.
>       * 10:8 NodesPerProcessor: Node per processor. Read-only. Reset: XXXb.
>       *      ValidValues:
> -     *      Value Description
> -     *      000b  1 node per processor.
> -     *      001b  2 nodes per processor.
> -     *      010b Reserved.
> -     *      011b 4 nodes per processor.
> -     *      111b-100b Reserved.
> +     *      Value   Description
> +     *      0h      1 node per processor.
> +     *      7h-1h   Reserved.
>       *  7:0 NodeId: Node ID. Read-only. Reset: XXh.
>       *
>       * NOTE: Hardware reserves 3 bits for number of nodes per processor.
> @@ -412,8 +409,12 @@ static void encode_topo_cpuid8000001e(X86CPU *cpu, 
> X86CPUTopoInfo *topo_info,
>       * NodeId is combination of node and socket_id which is already decoded
>       * in apic_id. Just use it by shifting.
>       */
> -    *ecx = ((topo_info->dies_per_pkg - 1) << 8) |
> -           ((cpu->apic_id >> apicid_die_offset(topo_info)) & 0xFF);
> +    if (cpu->legacy_multi_node) {
> +        *ecx = ((topo_info->dies_per_pkg - 1) << 8) |
> +               ((cpu->apic_id >> apicid_die_offset(topo_info)) & 0xFF);
> +    } else {
> +        *ecx = (cpu->apic_id >> apicid_pkg_offset(topo_info)) & 0xFF;
> +    }
>  
>      *edx = 0;
>  }
> @@ -7895,6 +7896,7 @@ static Property x86_cpu_properties[] = {
>       * own cache information (see x86_cpu_load_def()).
>       */
>      DEFINE_PROP_BOOL("legacy-cache", X86CPU, legacy_cache, true),
> +    DEFINE_PROP_BOOL("legacy-multi-node", X86CPU, legacy_multi_node, false),
>      DEFINE_PROP_BOOL("xen-vapic", X86CPU, xen_vapic, false),
>  
>      /*
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index ef987f344c..6ef4396fc5 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1989,6 +1989,12 @@ struct ArchCPU {
>       */
>      bool legacy_cache;
>  
> +    /* Compatibility bits for old machine types.
> +     * If true decode the CPUID Function 0x8000001E_ECX to support multiple
> +     * nodes per processor
> +     */
> +    bool legacy_multi_node;
> +
>      /* Compatibility bits for old machine types: */
>      bool enable_cpuid_0xb;
>  

-- 
Thanks
Babu Moger