Re: [Qemu-devel] [PATCH v1] kvm: Make vcpu->requests as 64 bit bitmap

[Roman Kagan]

On Thu, Dec 24, 2015 at 12:30:26PM +0300, Andrey Smetanin wrote:
> Currently on x86 arch we has already 32 requests defined
> so the newer request bits can't be placed inside
> vcpu->requests(unsigned long) inside x86 32 bit system.
> But we are going to add a new request in x86 arch
> for Hyper-V tsc page support.
> 
> To solve the problem the patch replaces vcpu->requests by
> bitmap with 64 bit length and uses bitmap API.
> 
> The patch consists of:
> * announce kvm_vcpu_has_requests() to check whether vcpu has
> requests
> * announce kvm_vcpu_requests() to get vcpu requests pointer

I think that if abstracting out the implementation of the request
container is what you're after, you'd better not define this function;
accesses to the request map should be through your accessor functions.

> * announce kvm_clear_request() to clear particular vcpu request
> * replace if (vcpu->requests) by if (kvm_vcpu_has_requests(vcpu))
> * replace clear_bit(req, vcpu->requests) by
>  kvm_clear_request(req, vcpu)

Apparently one accessor is missing: test the presence of a request
without clearing it from the bitmap (because kvm_check_request() clears
it).

> diff --git a/arch/powerpc/kvm/trace.h b/arch/powerpc/kvm/trace.h
> index 2e0e67e..4015b88 100644
> --- a/arch/powerpc/kvm/trace.h
> +++ b/arch/powerpc/kvm/trace.h
> @@ -109,7 +109,7 @@ TRACE_EVENT(kvm_check_requests,
>  
>       TP_fast_assign(
>               __entry->cpu_nr         = vcpu->vcpu_id;
> -             __entry->requests       = vcpu->requests;
> +             __entry->requests       = (__u32)kvm_vcpu_requests(vcpu)[0];
>       ),

This doesn't make sense, to expose only subset of the requests.  BTW I
don't see this event in Linus tree, nor in linux-next, so I'm not quite
sure why it's formed this way; I guess the interesting part is the
request number and the return value (i.e. whether it's set), not the
whole bitmap.

> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5957,7 +5957,7 @@ static int handle_invalid_guest_state(struct kvm_vcpu 
> *vcpu)
>               if (intr_window_requested && vmx_interrupt_allowed(vcpu))
>                       return handle_interrupt_window(&vmx->vcpu);
>  
> -             if (test_bit(KVM_REQ_EVENT, &vcpu->requests))
> +             if (test_bit(KVM_REQ_EVENT, kvm_vcpu_requests(vcpu)))

Here you'd rather want that function to test for the request without
clearing it.

> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2117,7 +2117,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct 
> msr_data *msr_info)
>  
>                       if (ka->boot_vcpu_runs_old_kvmclock != tmp)
>                               set_bit(KVM_REQ_MASTERCLOCK_UPDATE,
> -                                     &vcpu->requests);
> +                                     kvm_vcpu_requests(vcpu));

This should have been kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);

> @@ -8048,7 +8048,7 @@ static inline bool kvm_vcpu_has_events(struct kvm_vcpu 
> *vcpu)
>       if (atomic_read(&vcpu->arch.nmi_queued))
>               return true;
>  
> -     if (test_bit(KVM_REQ_SMI, &vcpu->requests))
> +     if (test_bit(KVM_REQ_SMI, kvm_vcpu_requests(vcpu)))

Again the test-only accessor is due here.

> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -146,6 +146,8 @@ static inline bool is_error_page(struct page *page)
>  #define KVM_REQ_HV_EXIT           30
>  #define KVM_REQ_HV_STIMER         31
>  
> +#define KVM_REQ_MAX               64
> +
>  #define KVM_USERSPACE_IRQ_SOURCE_ID          0
>  #define KVM_IRQFD_RESAMPLE_IRQ_SOURCE_ID     1
>  
> @@ -233,7 +235,7 @@ struct kvm_vcpu {
>       int vcpu_id;
>       int srcu_idx;
>       int mode;
> -     unsigned long requests;
> +     DECLARE_BITMAP(requests, KVM_REQ_MAX);
>       unsigned long guest_debug;
>  
>       int pre_pcpu;
> @@ -286,6 +288,16 @@ struct kvm_vcpu {
>       struct kvm_vcpu_arch arch;
>  };
>  
> +static inline bool kvm_vcpu_has_requests(struct kvm_vcpu *vcpu)
> +{
> +     return !bitmap_empty(vcpu->requests, KVM_REQ_MAX);
> +}
> +
> +static inline ulong *kvm_vcpu_requests(struct kvm_vcpu *vcpu)
> +{
> +     return (ulong *)vcpu->requests;
> +}

As I wrote above, I believe this function doesn't belong in the API.

> +
>  static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)
>  {
>       return cmpxchg(&vcpu->mode, IN_GUEST_MODE, EXITING_GUEST_MODE);
> @@ -1002,7 +1014,7 @@ static inline bool kvm_is_error_gpa(struct kvm *kvm, 
> gpa_t gpa)
>  
>  static inline void kvm_migrate_timers(struct kvm_vcpu *vcpu)
>  {
> -     set_bit(KVM_REQ_MIGRATE_TIMER, &vcpu->requests);
> +     set_bit(KVM_REQ_MIGRATE_TIMER, kvm_vcpu_requests(vcpu));

kvm_make_request(KVM_REQ_MIGRATE_TIMER, vcpu);

> @@ -1118,19 +1130,24 @@ static inline bool kvm_vcpu_compatible(struct 
> kvm_vcpu *vcpu) { return true; }
>  
>  static inline void kvm_make_request(int req, struct kvm_vcpu *vcpu)
>  {
> -     set_bit(req, &vcpu->requests);
> +     set_bit(req, kvm_vcpu_requests(vcpu));
>  }
>  
>  static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
>  {
> -     if (test_bit(req, &vcpu->requests)) {
> -             clear_bit(req, &vcpu->requests);
> +     if (test_bit(req, kvm_vcpu_requests(vcpu))) {
> +             clear_bit(req, kvm_vcpu_requests(vcpu));
>               return true;
>       } else {
>               return false;
>       }

I wonder why this doesn't use test_and_clear_bit instead.

Roman.