Re: [RFC 03/33] KVM: x86: hyper-v: Introduce XMM output support

[Alexander Graf <graf@amazon.com>]

On 08.11.23 13:11, Vitaly Kuznetsov wrote:
> Alexander Graf <graf@amazon.com> writes:
>
>> On 08.11.23 12:17, Nicolas Saenz Julienne wrote:
>>> Prepare infrastructure to be able to return data through the XMM
>>> registers when Hyper-V hypercalls are issues in fast mode. The XMM
>>> registers are exposed to user-space through KVM_EXIT_HYPERV_HCALL and
>>> restored on successful hypercall completion.
>>>
>>> Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
>>> ---
>>>    arch/x86/include/asm/hyperv-tlfs.h |  2 +-
>>>    arch/x86/kvm/hyperv.c              | 33 +++++++++++++++++++++++++++++-
>>>    include/uapi/linux/kvm.h           |  6 ++++++
>>>    3 files changed, 39 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-tlfs.h
>>> index 2ff26f53cd62..af594aa65307 100644
>>> --- a/arch/x86/include/asm/hyperv-tlfs.h
>>> +++ b/arch/x86/include/asm/hyperv-tlfs.h
>>> @@ -49,7 +49,7 @@
>>>    /* Support for physical CPU dynamic partitioning events is available*/
>>>    #define HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE  BIT(3)
>>>    /*
>>> - * Support for passing hypercall input parameter block via XMM
>>> + * Support for passing hypercall input and output parameter block via XMM
>>>     * registers is available
>>>     */
>>>    #define HV_X64_HYPERCALL_XMM_INPUT_AVAILABLE               BIT(4)
>>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>>> index 238afd7335e4..e1bc861ab3b0 100644
>>> --- a/arch/x86/kvm/hyperv.c
>>> +++ b/arch/x86/kvm/hyperv.c
>>> @@ -1815,6 +1815,7 @@ struct kvm_hv_hcall {
>>>       u16 rep_idx;
>>>       bool fast;
>>>       bool rep;
>>> +    bool xmm_dirty;
>>>       sse128_t xmm[HV_HYPERCALL_MAX_XMM_REGISTERS];
>>>
>>>       /*
>>> @@ -2346,9 +2347,33 @@ static int kvm_hv_hypercall_complete(struct kvm_vcpu *vcpu, u64 result)
>>>       return ret;
>>>    }
>>>
>>> +static void kvm_hv_write_xmm(struct kvm_hyperv_xmm_reg *xmm)
>>> +{
>>> +    int reg;
>>> +
>>> +    kvm_fpu_get();
>>> +    for (reg = 0; reg < HV_HYPERCALL_MAX_XMM_REGISTERS; reg++) {
>>> +            const sse128_t data = sse128(xmm[reg].low, xmm[reg].high);
>>> +            _kvm_write_sse_reg(reg, &data);
>>> +    }
>>> +    kvm_fpu_put();
>>> +}
>>> +
>>> +static bool kvm_hv_is_xmm_output_hcall(u16 code)
>>> +{
>>> +    return false;
>>> +}
>>> +
>>>    static int kvm_hv_hypercall_complete_userspace(struct kvm_vcpu *vcpu)
>>>    {
>>> -    return kvm_hv_hypercall_complete(vcpu, vcpu->run->hyperv.u.hcall.result);
>>> +    bool fast = !!(vcpu->run->hyperv.u.hcall.input & HV_HYPERCALL_FAST_BIT);
>>> +    u16 code = vcpu->run->hyperv.u.hcall.input & 0xffff;
>>> +    u64 result = vcpu->run->hyperv.u.hcall.result;
>>> +
>>> +    if (kvm_hv_is_xmm_output_hcall(code) && hv_result_success(result) && fast)
>>> +            kvm_hv_write_xmm(vcpu->run->hyperv.u.hcall.xmm);
>>> +
>>> +    return kvm_hv_hypercall_complete(vcpu, result);
>>>    }
>>>
>>>    static u16 kvm_hvcall_signal_event(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
>>> @@ -2623,6 +2648,9 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>>>               break;
>>>       }
>>>
>>> +    if ((ret & HV_HYPERCALL_RESULT_MASK) == HV_STATUS_SUCCESS && hc.xmm_dirty)
>>> +            kvm_hv_write_xmm((struct kvm_hyperv_xmm_reg*)hc.xmm);
>>> +
>>>    hypercall_complete:
>>>       return kvm_hv_hypercall_complete(vcpu, ret);
>>>
>>> @@ -2632,6 +2660,8 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>>>       vcpu->run->hyperv.u.hcall.input = hc.param;
>>>       vcpu->run->hyperv.u.hcall.params[0] = hc.ingpa;
>>>       vcpu->run->hyperv.u.hcall.params[1] = hc.outgpa;
>>> +    if (hc.fast)
>>> +            memcpy(vcpu->run->hyperv.u.hcall.xmm, hc.xmm, sizeof(hc.xmm));
>>>       vcpu->arch.complete_userspace_io = kvm_hv_hypercall_complete_userspace;
>>>       return 0;
>>>    }
>>> @@ -2780,6 +2810,7 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
>>>                       ent->ebx |= HV_ENABLE_EXTENDED_HYPERCALLS;
>>>
>>>                       ent->edx |= HV_X64_HYPERCALL_XMM_INPUT_AVAILABLE;
>>> +                    ent->edx |= HV_X64_HYPERCALL_XMM_OUTPUT_AVAILABLE;
>>
>> Shouldn't this be guarded by an ENABLE_CAP to make sure old user space
>> that doesn't know about xmm outputs is still able to run with newer kernels?
>>
> No, we don't do CAPs for new Hyper-V features anymore since we have
> KVM_GET_SUPPORTED_HV_CPUID. Userspace is not supposed to simply copy
> its output into guest visible CPUIDs, it must only enable features it
> knows. Even 'hv_passthrough' option in QEMU doesn't pass unknown
> features through.


Ah, nice :). That simplifies things.


Alex




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