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

[Zhao Liu <zhao1.liu@intel.com>]

On Fri, Nov 10, 2023 at 11:08:06AM -0600, Babu Moger wrote:
> Date: Fri, 10 Nov 2023 11:08:06 -0600
> From: Babu Moger <babu.moger@amd.com>
> Subject: [PATCH] target/i386: Fix CPUID encoding of Fn8000001E_ECX
> X-Mailer: git-send-email 2.34.1
> 
> 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. 

One additional query, such dependency relationship is not reflected in
encode_topo_cpuid8000001e(), should TOPOEXT be checked in
encode_topo_cpuid8000001e()?

> 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>
> ---

[snip]

> +++ b/target/i386/cpu.h
> @@ -1988,6 +1988,7 @@ struct ArchCPU {
>       * If true present the old cache topology information
>       */
>      bool legacy_cache;
> +    bool legacy_multi_node;

This property deserves a comment, as does legacy_cache above.

>  
>      /* Compatibility bits for old machine types: */
>      bool enable_cpuid_0xb;
> -- 
> 2.34.1
> 

Just the above nit, otherwise,
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>