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Date: Wed, 09 Dec 2020 10:27:21 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Ankur Arora <ankur.a.arora@...cle.com>,
Joao Martins <joao.m.martins@...cle.com>, karahmed@...zon.de
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Radim Krčmář <rkrcmar@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 10/39] KVM: x86/xen: support upcall vector
On Tue, 2020-12-08 at 22:35 -0800, Ankur Arora wrote:
> > It looks like Linux doesn't use the per-vCPU upcall vector that you
> > called 'KVM_XEN_CALLBACK_VIA_EVTCHN'. So I'm delivering interrupts via
> > KVM_INTERRUPT as if they were ExtINT....
> >
> > ... except I'm not. Because the kernel really does expect that to be an
> > ExtINT from a legacy PIC, and kvm_apic_accept_pic_intr() only returns
> > true if LVT0 is set up for EXTINT and unmasked.
> >
> > I messed around with this hack and increasingly desperate variations on
> > the theme (since this one doesn't cause the IRQ window to be opened to
> > userspace in the first place), but couldn't get anything working:
>
> Increasingly desperate variations, about sums up my process as well while
> trying to get the upcall vector working.
:)
So this seems to work, and I think it's about as minimal as it can get.
All it does is implement a kvm_xen_has_interrupt() which checks the
vcpu_info->evtchn_upcall_pending flag, just like Xen does.
With this, my completely userspace implementation of event channels is
working. And of course this forms a basis for adding the minimal
acceleration of IPI/VIRQ that we need to the kernel, too.
My only slight concern is the bit in vcpu_enter_guest() where we have
to add the check for kvm_xen_has_interrupt(), because nothing is
setting KVM_REQ_EVENT. I suppose I could look at having something —
even an explicit ioctl from userspace — set that for us.... BUT...
I'm not sure that condition wasn't *already* broken some cases for
KVM_INTERRUPT anyway. In kvm_vcpu_ioctl_interrupt() we set
vcpu->arch.pending_userspace_vector and we *do* request KVM_REQ_EVENT,
sure.
But what if vcpu_enter_guest() processes that the first time (and
clears KVM_REQ_EVENT), and then exits for some *other* reason with
interrupts still disabled? Next time through vcpu_enter_guest(), even
though kvm_cpu_has_injectable_intr() is still true, we don't enable the
IRQ windowvmexit because KVM_REQ_EVENT got lost so we don't even call
inject_pending_event().
So instead of just kvm_xen_has_interrupt() in my patch below, I wonder
if we need to use kvm_cpu_has_injectable_intr() to fix the existing
bug? Or am I missing something there and there isn't a bug after all?
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index d8716ef27728..4a63f212fdfc 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -902,6 +902,7 @@ struct msr_bitmap_range {
/* Xen emulation context */
struct kvm_xen {
bool long_mode;
+ u8 upcall_vector;
struct kvm_host_map shinfo_map;
void *shinfo;
};
diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index 814698e5b152..24668b51b5c8 100644
--- a/arch/x86/kvm/irq.c
+++ b/arch/x86/kvm/irq.c
@@ -14,6 +14,7 @@
#include "irq.h"
#include "i8254.h"
#include "x86.h"
+#include "xen.h"
/*
* check if there are pending timer events
@@ -56,6 +57,9 @@ int kvm_cpu_has_extint(struct kvm_vcpu *v)
if (!lapic_in_kernel(v))
return v->arch.interrupt.injected;
+ if (kvm_xen_has_interrupt(v))
+ return 1;
+
if (!kvm_apic_accept_pic_intr(v))
return 0;
@@ -110,6 +114,9 @@ static int kvm_cpu_get_extint(struct kvm_vcpu *v)
if (!lapic_in_kernel(v))
return v->arch.interrupt.nr;
+ if (kvm_xen_has_interrupt(v))
+ return v->kvm->arch.xen.upcall_vector;
+
if (irqchip_split(v->kvm)) {
int vector = v->arch.pending_external_vector;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ad9eea8f4f26..1711072b3616 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8891,7 +8891,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
kvm_x86_ops.msr_filter_changed(vcpu);
}
- if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
+ if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win ||
+ kvm_xen_has_interrupt(vcpu)) {
++vcpu->stat.req_event;
kvm_apic_accept_events(vcpu);
if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 4aa776c1ad57..116422d93d96 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -257,6 +257,22 @@ void kvm_xen_setup_pvclock_page(struct kvm_vcpu *v)
srcu_read_unlock(&v->kvm->srcu, idx);
}
+int kvm_xen_has_interrupt(struct kvm_vcpu *v)
+{
+ int rc = 0;
+
+ if (v->kvm->arch.xen.upcall_vector) {
+ int idx = srcu_read_lock(&v->kvm->srcu);
+ struct vcpu_info *vcpu_info = xen_vcpu_info(v);
+
+ if (vcpu_info && READ_ONCE(vcpu_info->evtchn_upcall_pending))
+ rc = 1;
+
+ srcu_read_unlock(&v->kvm->srcu, idx);
+ }
+ return rc;
+}
+
static int vcpu_attr_loc(struct kvm_vcpu *vcpu, u16 type,
struct kvm_host_map **map, void ***hva, size_t *sz)
{
@@ -338,6 +354,14 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
break;
}
+ case KVM_XEN_ATTR_TYPE_UPCALL_VECTOR:
+ if (data->u.vector < 0x10)
+ return -EINVAL;
+
+ kvm->arch.xen.upcall_vector = data->u.vector;
+ r = 0;
+ break;
+
default:
break;
}
@@ -386,6 +410,11 @@ int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
break;
}
+ case KVM_XEN_ATTR_TYPE_UPCALL_VECTOR:
+ data->u.vector = kvm->arch.xen.upcall_vector;
+ r = 0;
+ break;
+
default:
break;
}
diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index ccd6002f55bc..1f599342f02c 100644
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -25,6 +25,7 @@ static inline struct kvm_vcpu *xen_vcpu_to_vcpu(struct kvm_vcpu_xen *xen_vcpu)
void kvm_xen_setup_pvclock_page(struct kvm_vcpu *vcpu);
void kvm_xen_setup_runstate_page(struct kvm_vcpu *vcpu);
void kvm_xen_runstate_set_preempted(struct kvm_vcpu *vcpu);
+int kvm_xen_has_interrupt (struct kvm_vcpu *vcpu);
int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
int kvm_xen_hvm_get_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
int kvm_xen_hypercall(struct kvm_vcpu *vcpu);
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 1047364d1adf..113279fa9e1e 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1587,6 +1587,7 @@ struct kvm_xen_hvm_attr {
union {
__u8 long_mode;
+ __u8 vector;
struct {
__u64 gfn;
} shared_info;
@@ -1604,6 +1605,7 @@ struct kvm_xen_hvm_attr {
#define KVM_XEN_ATTR_TYPE_VCPU_INFO 0x2
#define KVM_XEN_ATTR_TYPE_VCPU_TIME_INFO 0x3
#define KVM_XEN_ATTR_TYPE_VCPU_RUNSTATE 0x4
+#define KVM_XEN_ATTR_TYPE_UPCALL_VECTOR 0x5
/* Secure Encrypted Virtualization command */
enum sev_cmd_id {
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