Re: [PATCH v3A 1/6] target/i386: add support for FRED in CPUID enumeration

[Zhao Liu]

On Thu, Dec 21, 2023 at 07:03:36PM -0800, Xin Li wrote:
> Date: Thu, 21 Dec 2023 19:03:36 -0800
> From: Xin Li <xin3.li@intel.com>
> Subject: [PATCH v3A 1/6] target/i386: add support for FRED in CPUID
>  enumeration
> X-Mailer: git-send-email 2.43.0
> 
> FRED, i.e., the Intel flexible return and event delivery architecture,
> defines simple new transitions that change privilege level (ring
> transitions).
> 
> The new transitions defined by the FRED architecture are FRED event
> delivery and, for returning from events, two FRED return instructions.
> FRED event delivery can effect a transition from ring 3 to ring 0, but
> it is used also to deliver events incident to ring 0.  One FRED
> instruction (ERETU) effects a return from ring 0 to ring 3, while the
> other (ERETS) returns while remaining in ring 0.  Collectively, FRED
> event delivery and the FRED return instructions are FRED transitions.
> 
> In addition to these transitions, the FRED architecture defines a new
> instruction (LKGS) for managing the state of the GS segment register.
> The LKGS instruction can be used by 64-bit operating systems that do
> not use the new FRED transitions.
> 
> WRMSRNS is an instruction that behaves exactly like WRMSR, with the
> only difference being that it is not a serializing instruction by
> default.  Under certain conditions, WRMSRNS may replace WRMSR to improve
> performance.  FRED uses it to switch RSP0 in a faster manner.
> 
> Search for the latest FRED spec in most search engines with this search
> pattern:
> 
>   site:intel.com FRED (flexible return and event delivery) specification
> 
> The CPUID feature flag CPUID.(EAX=7,ECX=1):EAX[17] enumerates FRED, and
> the CPUID feature flag CPUID.(EAX=7,ECX=1):EAX[18] enumerates LKGS, and
> the CPUID feature flag CPUID.(EAX=7,ECX=1):EAX[19] enumerates WRMSRNS.
> 
> Add CPUID definitions for FRED/LKGS/WRMSRNS, and expose them to KVM guests.
> 
> Because FRED relies on LKGS and WRMSRNS, add that to feature dependency
> map.
> 
> Tested-by: Shan Kang <shan.kang@intel.com>
> Signed-off-by: Xin Li <xin3.li@intel.com>
> ---

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>

> 
> Changelog
> v3A:
> - Fix reversed dependency (Wu Dan1).
> ---
>  target/i386/cpu.c | 10 +++++++++-
>  target/i386/cpu.h |  6 ++++++
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 358d9c0a65..66551c7eae 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -965,7 +965,7 @@ FeatureWordInfo feature_word_info[FEATURE_WORDS] = {
>              "avx-vnni", "avx512-bf16", NULL, "cmpccxadd",
>              NULL, NULL, "fzrm", "fsrs",
>              "fsrc", NULL, NULL, NULL,
> -            NULL, NULL, NULL, NULL,
> +            NULL, "fred", "lkgs", "wrmsrns",
>              NULL, "amx-fp16", NULL, "avx-ifma",
>              NULL, NULL, NULL, NULL,
>              NULL, NULL, NULL, NULL,
> @@ -1552,6 +1552,14 @@ static FeatureDep feature_dependencies[] = {
>          .from = { FEAT_VMX_SECONDARY_CTLS,  
> VMX_SECONDARY_EXEC_ENABLE_USER_WAIT_PAUSE },
>          .to = { FEAT_7_0_ECX,               CPUID_7_0_ECX_WAITPKG },
>      },
> +    {
> +        .from = { FEAT_7_1_EAX,             CPUID_7_1_EAX_LKGS },
> +        .to = { FEAT_7_1_EAX,               CPUID_7_1_EAX_FRED },
> +    },
> +    {
> +        .from = { FEAT_7_1_EAX,             CPUID_7_1_EAX_WRMSRNS },
> +        .to = { FEAT_7_1_EAX,               CPUID_7_1_EAX_FRED },
> +    },
>  };
>  
>  typedef struct X86RegisterInfo32 {
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index cd2e295bd6..5faf00551d 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -934,6 +934,12 @@ uint64_t x86_cpu_get_supported_feature_word(FeatureWord 
> w,
>  #define CPUID_7_1_EDX_AMX_COMPLEX       (1U << 8)
>  /* PREFETCHIT0/1 Instructions */
>  #define CPUID_7_1_EDX_PREFETCHITI       (1U << 14)
> +/* Flexible return and event delivery (FRED) */
> +#define CPUID_7_1_EAX_FRED              (1U << 17)
> +/* Load into IA32_KERNEL_GS_BASE (LKGS) */
> +#define CPUID_7_1_EAX_LKGS              (1U << 18)
> +/* Non-Serializing Write to Model Specific Register (WRMSRNS) */
> +#define CPUID_7_1_EAX_WRMSRNS           (1U << 19)
>  
>  /* Do not exhibit MXCSR Configuration Dependent Timing (MCDT) behavior */
>  #define CPUID_7_2_EDX_MCDT_NO           (1U << 5)
> -- 
> 2.43.0
> 
>