linux-kernel - Re: [RFC PATCH v6 4/6] KVM: x86: Let userspace re-enable previously disabled exits

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Date:   Mon, 30 Jan 2023 14:19:03 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Kechen Lu <kechenl@...dia.com>
CC:     <kvm@...r.kernel.org>, <seanjc@...gle.com>, <pbonzini@...hat.com>,
        <zhi.wang.linux@...il.com>, <shaoqin.huang@...el.com>,
        <vkuznets@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v6 4/6] KVM: x86: Let userspace re-enable previously
 disabled exits

On Sat, Jan 21, 2023 at 02:07:36AM +0000, Kechen Lu wrote:
>From: Sean Christopherson <seanjc@...gle.com>
>
>Add an OVERRIDE flag to KVM_CAP_X86_DISABLE_EXITS allow userspace to
>re-enable exits and/or override previous settings.  There's no real use
>case for the per-VM ioctl, but a future per-vCPU variant wants to let
>userspace toggle interception while the vCPU is running; add the OVERRIDE
>functionality now to provide consistent between the per-VM and
>per-vCPU variants.
>
>Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Kechen, add your signed-off-by as you are the submitter.

>---
> Documentation/virt/kvm/api.rst |  5 +++++
> arch/x86/kvm/x86.c             | 32 ++++++++++++++++++++++++--------
> include/uapi/linux/kvm.h       |  4 +++-
> 3 files changed, 32 insertions(+), 9 deletions(-)
>
>diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>index fb0fcc566d5a..3850202942d0 100644
>--- a/Documentation/virt/kvm/api.rst
>+++ b/Documentation/virt/kvm/api.rst
>@@ -7095,6 +7095,7 @@ Valid bits in args[0] are::
>   #define KVM_X86_DISABLE_EXITS_HLT              (1 << 1)
>   #define KVM_X86_DISABLE_EXITS_PAUSE            (1 << 2)
>   #define KVM_X86_DISABLE_EXITS_CSTATE           (1 << 3)
>+  #define KVM_X86_DISABLE_EXITS_OVERRIDE         (1ull << 63)
> 
> Enabling this capability on a VM provides userspace with a way to no
> longer intercept some instructions for improved latency in some
>@@ -7103,6 +7104,10 @@ physical CPUs.  More bits can be added in the future; userspace can
> just pass the KVM_CHECK_EXTENSION result to KVM_ENABLE_CAP to disable
> all such vmexits.
> 
>+By default, this capability only disables exits.  To re-enable an exit, or to
>+override previous settings, userspace can set KVM_X86_DISABLE_EXITS_OVERRIDE,
>+in which case KVM will enable/disable according to the mask (a '1' == disable).
>+
> Do not enable KVM_FEATURE_PV_UNHALT if you disable HLT exits.
> 
> 7.14 KVM_CAP_S390_HPAGE_1M
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 60caa3fd40e5..3ea5f12536a0 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -5484,6 +5484,28 @@ static int kvm_vcpu_ioctl_device_attr(struct kvm_vcpu *vcpu,
> 	return r;
> }
> 
>+
>+#define kvm_ioctl_disable_exits(a, mask)				     \
>+({									     \

>+	if (!kvm_can_mwait_in_guest())                                       \
>+		(mask) &= KVM_X86_DISABLE_EXITS_MWAIT;                       \

This can be dropped or should be a WARN_ON_ONCE() because if kvm cannot
support mwait in guest (i.e., !kvm_can_mwait_in_guest()), attempts to
disable mwait exits are already treated as invalid requests in patch 3.

>+	if ((mask) & KVM_X86_DISABLE_EXITS_OVERRIDE) {			     \
>+		(a).mwait_in_guest = (mask) & KVM_X86_DISABLE_EXITS_MWAIT;   \
>+		(a).hlt_in_guest = (mask) & KVM_X86_DISABLE_EXITS_HLT;	     \
>+		(a).pause_in_guest = (mask) & KVM_X86_DISABLE_EXITS_PAUSE;   \
>+		(a).cstate_in_guest = (mask) & KVM_X86_DISABLE_EXITS_CSTATE; \
>+	} else {							     \
>+		if ((mask) & KVM_X86_DISABLE_EXITS_MWAIT)		     \
>+			(a).mwait_in_guest = true;			     \
>+		if ((mask) & KVM_X86_DISABLE_EXITS_HLT)			     \
>+			(a).hlt_in_guest = true;			     \
>+		if ((mask) & KVM_X86_DISABLE_EXITS_PAUSE)		     \
>+			(a).pause_in_guest = true;			     \
>+		if ((mask) & KVM_X86_DISABLE_EXITS_CSTATE)		     \
>+			(a).cstate_in_guest = true;			     \
>+	}								     \
>+})
>+
> static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
> 				     struct kvm_enable_cap *cap)
> {
>@@ -6238,14 +6260,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> 		if (kvm->created_vcpus)
> 			goto disable_exits_unlock;
> 
>-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT)
>-			kvm->arch.mwait_in_guest = true;
>-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_HLT)
>-			kvm->arch.hlt_in_guest = true;
>-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
>-			kvm->arch.pause_in_guest = true;
>-		if (cap->args[0] & KVM_X86_DISABLE_EXITS_CSTATE)
>-			kvm->arch.cstate_in_guest = true;
>+		kvm_ioctl_disable_exits(kvm->arch, cap->args[0]);
>+
> 		r = 0;
> disable_exits_unlock:
> 		mutex_unlock(&kvm->lock);
>diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>index 55155e262646..876dcccbfff2 100644
>--- a/include/uapi/linux/kvm.h
>+++ b/include/uapi/linux/kvm.h
>@@ -823,10 +823,12 @@ struct kvm_ioeventfd {
> #define KVM_X86_DISABLE_EXITS_HLT            (1 << 1)
> #define KVM_X86_DISABLE_EXITS_PAUSE          (1 << 2)
> #define KVM_X86_DISABLE_EXITS_CSTATE         (1 << 3)
>+#define KVM_X86_DISABLE_EXITS_OVERRIDE	     (1ull << 63)
> #define KVM_X86_DISABLE_VALID_EXITS          (KVM_X86_DISABLE_EXITS_MWAIT | \
>                                               KVM_X86_DISABLE_EXITS_HLT | \
>                                               KVM_X86_DISABLE_EXITS_PAUSE | \
>-                                              KVM_X86_DISABLE_EXITS_CSTATE)
>+					      KVM_X86_DISABLE_EXITS_CSTATE | \
>+					      KVM_X86_DISABLE_EXITS_OVERRIDE)
> 
> /* for KVM_ENABLE_CAP */
> struct kvm_enable_cap {
>-- 
>2.34.1
>