Re: [PATCH RFC 10/39] KVM: x86/xen: support upcall vector

[Joao Martins]

On 12/9/20 10:27 AM, David Woodhouse wrote:
> 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...
> 
Isn't this what the first half of this patch was doing initially (minus the
irq routing) ? Looks really similar:

https://lore.kernel.org/kvm/20190220201609.28290-11-joao.m.martins@xxxxxxxxxx/

Albeit, I gotta after seeing the irq routing removed it ends much simpler, if we just
replace the irq routing with a domain-wide upcall vector ;)

Albeit it wouldn't cover the Windows Xen open source drivers which use the EVTCHN method
(which is a regular LAPIC vector) [to answer your question on what uses it] For the EVTCHN
you can just inject a regular vector through lapic deliver, and guest acks it. Sadly Linux
doesn't use it,
and if it was the case we would probably get faster upcalls with APICv/AVIC.

> 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?
> 
Given that the notion of an event channel pending is Xen specific handling, I am not sure
we can remove the kvm_xen_has_interrupt()/kvm_xen_get_interrupt() logic. Much of the
reason that I ended up checking on vmenter that checks event channels pendings..

That or the autoEOI hack Ankur and you were talking out.

> 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 {
>