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Message-ID: <3d7070d51dd0094e426b420bc5e7d09657dd8d38.camel@infradead.org>
Date: Tue, 19 Sep 2023 15:18:39 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Paul Durrant <paul@....org>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Cc: Paul Durrant <pdurrant@...zon.com>,
Sean Christopherson <seanjc@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v4 09/13] KVM: xen: automatically use the vcpu_info
embedded in shared_info
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?
> + 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?
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?
> + 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.
> + 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);
>
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