Re: [PATCH v4 09/13] KVM: xen: automatically use the vcpu_info embedded in shared_info
From: Paul Durrant
Date: Tue Sep 19 2023 - 10:34:12 EST
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);