linux-kernel - Re: [PATCH v2 11/49] KVM: x86: Disallow KVM_CAP_X86_DISABLE

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Message-ID: <dc19d74e25b9e7e42c693a13b6f98565fb799734.camel@redhat.com>
Date: Thu, 04 Jul 2024 21:17:02 -0400
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>,  Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Hou Wenlong
 <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>, Oliver Upton
 <oliver.upton@...ux.dev>, Binbin Wu <binbin.wu@...ux.intel.com>, Yang
 Weijiang <weijiang.yang@...el.com>, Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 11/49] KVM: x86: Disallow KVM_CAP_X86_DISABLE_EXITS
 after vCPU creation

On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> Reject KVM_CAP_X86_DISABLE_EXITS if vCPUs have been created, as disabling
> PAUSE/MWAIT/HLT exits after vCPUs have been created is broken and useless,
> e.g. except for PAUSE on SVM, the relevant intercepts aren't updated after
> vCPU creation.  vCPUs may also end up with an inconsistent configuration
> if exits are disabled between creation of multiple vCPUs.

Hi,

I am not sure that PAUSE intercepts are updated either, I wasn't able to find a code
that does this.

I agree with this change, but note that there was some talk on the mailing
list to allow to selectively disable VM exits (e.g PAUSE, MWAIT, ...) only on some vCPUs, 
based on the claim that some vCPUs might run RT tasks, while some might be housekeeping.
I haven't followed those discussions closely.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky

> 
> Cc: Hou Wenlong <houwenlong.hwl@...group.com>
> Link: https://lore.kernel.org/all/9227068821b275ac547eb2ede09ec65d2281fe07.1680179693.git.houwenlong.hwl@antgroup.com
> Link: https://lore.kernel.org/all/20230121020738.2973-2-kechenl@nvidia.com
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  Documentation/virt/kvm/api.rst | 1 +
>  arch/x86/kvm/x86.c             | 6 ++++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 6ab8b5b7c64e..884846282d06 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7645,6 +7645,7 @@ branch to guests' 0x200 interrupt vector.
>  :Architectures: x86
>  :Parameters: args[0] defines which exits are disabled
>  :Returns: 0 on success, -EINVAL when args[0] contains invalid exits
> +          or if any vCPUs have already been created
>  
>  Valid bits in args[0] are::
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index bb34891d2f0a..4cb0c150a2f8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6568,6 +6568,10 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
>  			break;
>  
> +		mutex_lock(&kvm->lock);
> +		if (kvm->created_vcpus)
> +			goto disable_exits_unlock;
> +
>  		if (cap->args[0] & KVM_X86_DISABLE_EXITS_PAUSE)
>  			kvm->arch.pause_in_guest = true;
>  
> @@ -6589,6 +6593,8 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>  		}
>  
>  		r = 0;
> +disable_exits_unlock:
> +		mutex_unlock(&kvm->lock);
>  		break;
>  	case KVM_CAP_MSR_PLATFORM_INFO:
>  		kvm->arch.guest_can_read_msr_platform_info = cap->args[0];