Re: [PATCH v4 09/13] KVM: xen: automatically use the vcpu_info embedded in shared_info

[Paul Durrant <xadimgnik@gmail.com>]

On 19/09/2023 15:18, David Woodhouse wrote:
> On Tue, 2023-09-19 at 13:41 +0000, Paul Durrant wrote:
>> --- a/arch/x86/kvm/xen.c
>> +++ b/arch/x86/kvm/xen.c
>> @@ -491,6 +491,21 @@ static void kvm_xen_inject_vcpu_vector(struct kvm_vcpu *v)
>>   
>>   static struct gfn_to_pfn_cache *get_vcpu_info_cache(struct kvm_vcpu *v, unsigned long *offset)
>>   {
>> +       if (!v->arch.xen.vcpu_info_cache.active && v->arch.xen.vcpu_id < MAX_VIRT_CPUS) {
>> +               struct kvm *kvm = v->kvm;
>> +
>> +               if (offset) {
>> +                       if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode)
>> +                               *offset = offsetof(struct shared_info,
>> +                                                  vcpu_info[v->arch.xen.vcpu_id]);
>> +                       else
>> +                               *offset = offsetof(struct compat_shared_info,
>> +                                                  vcpu_info[v->arch.xen.vcpu_id]);
>> +               }
>> +
>> +               return &kvm->arch.xen.shinfo_cache;
>> +       }
>> +
>>          if (offset)
>>                  *offset = 0;
>>   
>> @@ -764,6 +779,92 @@ static int kvm_xen_set_vcpu_id(struct kvm_vcpu *vcpu, unsigned int vcpu_id)
>>          return 0;
>>   }
>>   
>> +static int kvm_xen_set_vcpu_info(struct kvm_vcpu *vcpu, gpa_t gpa)
>> +{
>> +       struct kvm *kvm = vcpu->kvm;
>> +       struct gfn_to_pfn_cache *si_gpc = &kvm->arch.xen.shinfo_cache;
>> +       struct gfn_to_pfn_cache *vi_gpc = &vcpu->arch.xen.vcpu_info_cache;
>> +       unsigned long flags;
>> +       unsigned long offset;
>> +       int ret;
>> +
>> +       if (gpa == KVM_XEN_INVALID_GPA) {
>> +               kvm_gpc_deactivate(vi_gpc);
>> +               return 0;
>> +       }
>> +
>> +       /*
>> +        * In Xen it is not possible for an explicit vcpu_info to be set
>> +        * before the shared_info exists since the former is done in response
>> +        * to a hypercall and the latter is set up as part of domain creation.
>> +        * The first 32 vCPUs have a default vcpu_info embedded in shared_info
>> +        * the content of which is copied across when an explicit vcpu_info is
>> +        * set, which can also clearly not be done if we don't know where the
>> +        * shared_info is. Hence we need to enforce that the shared_info cache
>> +        * is active here.
>> +        */
>> +       if (!si_gpc->active)
>> +               return -EINVAL;
>> +
>> +       /* Setting an explicit vcpu_info is a one-off operation */
>> +       if (vi_gpc->active)
>> +               return -EINVAL;
> 
> Is that the errno that Xen will return to the hypercall if a guest
> tries it? I.e. if the VMM simply returns the errno that it gets from
> the kernel, is that OK?
> 

Yes, I checked. Xen returns -EINVAL.

>> +       ret = kvm_gpc_activate(vi_gpc, gpa, sizeof(struct vcpu_info));
> 
>  From this moment, can't interrupts be delivered to the new vcpu_info,
> even though the memcpy hasn't happened yet?
> 

Hmm, that's a good point. TBH it would be nice to have an 'activate and 
leave locked' primitive to avoid this.

> I think we need to ensure that any kvm_xen_set_evtchn_fast() which
> happens at this point cannot proceed, and falls back to the slow path.
> 
> Can we set a flag before we activate the vcpu_info and clear it after
> the memcpy is done, then make kvm_xen_set_evtchn_fast() return
> EWOULDBLOCK whenever that flag is set?
> 
> The slow path in kvm_xen_set_evtchn() takes kvm->arch.xen.xen_lock and
> I think kvm_xen_vcpu_set_attr() has taken that same lock before you get
> to this code, so it works out nicely?
> 

Yes, I think that is safe... but if we didn't have the window between 
activating the vcpu_info cache and doing the copy we'd also be ok I 
think... Or perhaps we could simply preserve evtchn_pending_sel and copy 
the rest of it?

> 
> 
>> +       if (ret)
>> +               return ret;
>> +
>> +       /* Nothing more to do if the vCPU is not among the first 32 */
>> +       if (vcpu->arch.xen.vcpu_id >= MAX_VIRT_CPUS)
>> +               return 0;
>> +
>> +       /*
>> +        * It's possible that the vcpu_info cache has been invalidated since
>> +        * we activated it so we need to go through the check-refresh dance.
>> +        */
>> +       read_lock_irqsave(&vi_gpc->lock, flags);
>> +       while (!kvm_gpc_check(vi_gpc, sizeof(struct vcpu_info))) {
>> +               read_unlock_irqrestore(&vi_gpc->lock, flags);
>> +
>> +               ret = kvm_gpc_refresh(vi_gpc, sizeof(struct vcpu_info));
>> +               if (ret) {
>> +                       kvm_gpc_deactivate(vi_gpc);
>> +                       return ret;
>> +               }
>> +
>> +               read_lock_irqsave(&vi_gpc->lock, flags);
>> +       }
>> +
>> +       /* Now lock the shared_info cache so we can copy the vcpu_info */
>> +       read_lock(&si_gpc->lock);
> 
> This adds a new lock ordering rule of the vcpu_info lock(s) before the
> shared_info lock. I don't know that it's *wrong* but it seems weird to
> me; I expected the shared_info to come first?
> 
> I avoided taking both at once in kvm_xen_set_evtchn_fast(), although
> maybe if we are going to have a rule that allows both, we could revisit
> that. Suspect it isn't needed.
> 
> Either way it is worth a clear comment somewhere to document the lock
> ordering, and I'd also like to know this has been tested with lockdep,
> which is often cleverer than me.
> 

Ok. I agree that shared_info before vcpu_info does seem more intuitive 
and maybe it would be better given the code in 
kvm_xen_set_evtchn_fast(). I'll seem how messy it gets in re-ordering 
and add a comment as you suggest.

   Paul

>> +       while (!kvm_gpc_check(si_gpc, PAGE_SIZE)) {
>> +               read_unlock(&si_gpc->lock);
>> +
>> +               ret = kvm_gpc_refresh(si_gpc, PAGE_SIZE);
>> +               if (ret) {
>> +                       read_unlock_irqrestore(&vi_gpc->lock, flags);
>> +                       kvm_gpc_deactivate(vi_gpc);
>> +                       return ret;
>> +               }
>> +
>> +               read_lock(&si_gpc->lock);
>> +       }
>> +
>> +       if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode)
>> +               offset = offsetof(struct shared_info,
>> +                                 vcpu_info[vcpu->arch.xen.vcpu_id]);
>> +       else
>> +               offset = offsetof(struct compat_shared_info,
>> +                                 vcpu_info[vcpu->arch.xen.vcpu_id]);
>> +
>> +       memcpy(vi_gpc->khva, si_gpc->khva + offset, sizeof(struct vcpu_info));
>> +
>> +       read_unlock(&si_gpc->lock);
>> +       read_unlock_irqrestore(&vi_gpc->lock, flags);
>> +
>> +       return 0;
>> +}
>> +
>>   int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>>   {
>>          int idx, r = -ENOENT;
>> @@ -779,14 +880,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
>>                  BUILD_BUG_ON(offsetof(struct vcpu_info, time) !=
>>                               offsetof(struct compat_vcpu_info, time));
>>   
>> -               if (data->u.gpa == KVM_XEN_INVALID_GPA) {
>> -                       kvm_gpc_deactivate(&vcpu->arch.xen.vcpu_info_cache);
>> -                       r = 0;
>> -                       break;
>> -               }
>> -
>> -               r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_info_cache,
>> -                                    data->u.gpa, sizeof(struct vcpu_info));
>> +               r = kvm_xen_set_vcpu_info(vcpu, data->u.gpa);
>>                  if (!r)
>>                          kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>>   
>