Re: [PATCH] target/i386: Return right size value after dynamic xfeature enabled

[Paolo Bonzini]

On 3/24/22 04:18, Yang Zhong wrote:
> The kvm_arch_get_supported_cpuid() only call KVM_GET_SUPPORTED_CPUID one
> time, so the cpuid buffer information still keep older value. Once Qemu
> enable new dynamic xfeature, like XTILEDATA, the cpuid[0D,0].{EBX,ECX}
> still return older value.
>
> This patch can return right size value in kvm_init_xsave() if XTILEDATA
> has been enabled by arch_prctl.
>
> assert(kvm_arch_get_supported_cpuid(kvm_state, 0xd, 0, R_ECX) <=
>             env->xsave_buf_len);
>
> Signed-off-by: Yang Zhong <yang.zhong@intel.com>

I don't understand, is this a bugfix for an assertion failure or just a 
cleanup?

Either way, while I like the idea of modifying 
kvm_arch_get_supported_cpuid, I think the right thing to do is to just 
use has_xsave2 as the return value if it is nonzero.  And then 
kvm_init_xsave can just do

if (!has_xsave) {
    return;
}
env->xsave_buf_len = kvm_arch_get_supported_cpuid(kvm_state, 0xd, 0, R_ECX);

without the assertion that is now obvious.

Paolo

> ---
>   target/i386/cpu.h     |  3 +++
>   target/i386/kvm/kvm.c | 15 +++++++++++++--
>   2 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 5e406088a9..814ba4020b 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -565,6 +565,9 @@ typedef enum X86Seg {
>   #define ESA_FEATURE_ALIGN64_MASK        (1U << ESA_FEATURE_ALIGN64_BIT)
>   #define ESA_FEATURE_XFD_MASK            (1U << ESA_FEATURE_XFD_BIT)
>   
> +#define ARCH_GET_XCOMP_GUEST_PERM       0x1024
> +#define ARCH_REQ_XCOMP_GUEST_PERM       0x1025
> +
>   
>   /* CPUID feature words */
>   typedef enum FeatureWord {
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index 06901c2a43..312d4fccf8 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -46,6 +46,7 @@
>   #include "hw/i386/intel_iommu.h"
>   #include "hw/i386/x86-iommu.h"
>   #include "hw/i386/e820_memory_layout.h"
> +#include "target/i386/cpu.h"
>   
>   #include "hw/pci/pci.h"
>   #include "hw/pci/msi.h"
> @@ -437,6 +438,18 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, 
> uint32_t function,
>               return ret;
>           }
>           ret = (reg == R_EAX) ? bitmask : bitmask >> 32;
> +    } else if (function == 0xd && index == 0 &&
> +               (reg == R_EBX || reg == R_ECX)) {
> +        /*
> +         * The value returned by KVM_GET_SUPPORTED_CPUID does not include
> +         * features that already be enabled with the arch_prctl system call.
> +         */
> +        int rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_GUEST_PERM, &bitmask);
> +        if (rc) {
> +            warn_report("prctl(ARCH_GET_XCOMP_GUEST_PERM) error: %d", rc);
> +        } else if (bitmask & XSTATE_XTILE_DATA_MASK) {
> +            ret += sizeof(XSaveXTILEDATA);
> +        }
>       } else if (function == 0x80000001 && reg == R_ECX) {
>           /*
>            * It's safe to enable TOPOEXT even if it's not returned by
> @@ -5214,8 +5227,6 @@ bool kvm_arch_cpu_check_are_resettable(void)
>       return !sev_es_enabled();
>   }
>   
> -#define ARCH_REQ_XCOMP_GUEST_PERM       0x1025
> -
>   void kvm_request_xsave_components(X86CPU *cpu, uint64_t mask)
>   {
>       KVMState *s = kvm_state;