Re: [PATCH 2/2] x86/cpuid: support LFENCE always serializing CPUID bit

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.04.2021 15:25, Andrew Cooper wrote:
> On 14/04/2021 14:05, Andrew Cooper wrote:
>> On 14/04/2021 13:57, Jan Beulich wrote:
>>> On 14.04.2021 13:04, Roger Pau Monne wrote:
>>>> @@ -264,6 +265,38 @@ struct cpuid_policy
>>>>              };
>>>>              uint32_t nc:8, :4, apic_id_size:4, :16;
>>>>              uint32_t /* d */:32;
>>>> +
>>>> +            uint64_t :64, :64; /* Leaf 0x80000009. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000000a - SVM rev and features. 
>>>> */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000000b. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000000c. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000000d. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000000e. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000000f. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000010. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000011. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000012. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000013. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000014. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000015. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000016. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000017. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000018. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000019 - TLB 1GB Identifiers. 
>>>> */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000001a - Performance related 
>>>> info. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000001b - IBS feature 
>>>> information. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000001c. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000001d - Cache properties. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000001e - Extd APIC/Core/Node 
>>>> IDs. */
>>>> +            uint64_t :64, :64; /* Leaf 0x8000001f - AMD Secure 
>>>> Encryption. */
>>>> +            uint64_t :64, :64; /* Leaf 0x80000020 - Platform QoS. */
>>>> +
>>>> +            /* Leaf 0x80000021 - Extended Feature 2 */
>>>> +            union {
>>>> +                uint32_t e21a;
>>>> +                struct { DECL_BITFIELD(e21a); };
>>>> +            };
>>>> +            uint32_t /* b */:32, /* c */:32, /* d */:32;
>>>>          };
>>>>      } extd;
>>> Due to the effect of this on what guests get to see, I think this
>>> wants to take my "x86/CPUID: shrink max_{,sub}leaf fields according
>>> to actual leaf contents" as a prereq, which in turn may better
>>> remain on top of "x86/CPUID: adjust extended leaves out of range
>>> clearing" (both are neighbors in that over 4 months old series,
>>> fair parts of which could imo go in irrespective of the unsettled
>>> dispute on xvmalloc() - unfortunately I had made that patch 2 of
>>> the series, not expecting it to be blocked for so long, and then
>>> presumably signaling to others that the rest of the series is also
>>> blocked).
>> There is no shrinking to be done in this case.  The bit is set across
>> the board on AMD/Hygon hardware, even on older parts.
>>
>> What does need changing however is the logic to trim max_extd_leaf down
>> to what hardware supports, so the bit is visible on Rome/older
>> hardware.  I.e. after this change, all VMs should get 0x80000021 by
>> default on AMD hardware.
>>
>> (A curious observation of Milan hardware is that it actually advertises
>> 0x80000023 as max_extd_leaf, and has two leaves of zeros at the end. 
>> I've got an open query about this.)
> 
> Something like this:
> 
> diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
> index 050cd5713e..d9eb2878c5 100644
> --- a/xen/arch/x86/cpuid.c
> +++ b/xen/arch/x86/cpuid.c
> @@ -311,6 +311,7 @@ static void __init calculate_raw_policy(void)
>  static void __init calculate_host_policy(void)
>  {
>      struct cpuid_policy *p = &host_cpuid_policy;
> +    unsigned int max_extd_leaf;
>  
>      *p = raw_cpuid_policy;
>  
> @@ -318,7 +319,18 @@ static void __init calculate_host_policy(void)
>          min_t(uint32_t, p->basic.max_leaf,   ARRAY_SIZE(p->basic.raw) - 1);
>      p->feat.max_subleaf =
>          min_t(uint32_t, p->feat.max_subleaf, ARRAY_SIZE(p->feat.raw) - 1);
> -    p->extd.max_leaf = 0x80000000 | min_t(uint32_t, p->extd.max_leaf &
> 0xffff,
> +
> +    max_extd_leaf = p->extd.max_leaf;
> +
> +    /*
> +     * For AMD/Hygon hardware before Zen3, we modify LFENCE to be dispatch
> +     * serialsing.  Extend max_extd_leaf beyond what hardware supports, to
> +     * include the feature leaf containing this information.
> +     */
> +    if ( cpu_has_lfence_dispatch )
> +        max_extd_leaf = max(max_extd_leaf, 0x80000021);
> +
> +    p->extd.max_leaf = 0x80000000 | min_t(uint32_t, max_extd_leaf & 0xffff,
>                                            ARRAY_SIZE(p->extd.raw) - 1);
>  
>      cpuid_featureset_to_policy(boot_cpu_data.x86_capability, p);

Well, why not set it to ARRAY_SIZE() and then have
x86_cpuid_policy_shrink_max_leaves() (from "x86/CPUID: shrink
max_{,sub}leaf fields according to actual leaf contents") have
a go? It'll recognize the non-zero leaf ... Otherwise, if we
gain a few more such special cases, things are going to get
ugly here.

Jan