linux-kernel - Re: [PATCH] x86/kvm/nVMCS: fix VMCLEAR when Enlightened VMCS is in use

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Date:   Tue, 25 Jun 2019 14:01:36 +0300
From:   Liran Alon <liran.alon@...cle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] x86/kvm/nVMCS: fix VMCLEAR when Enlightened VMCS is in
 use



> On 25 Jun 2019, at 11:51, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> Liran Alon <liran.alon@...cle.com> writes:
> 
>>> On 24 Jun 2019, at 16:30, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>> 
>>> 
>>> +bool nested_enlightened_vmentry(struct kvm_vcpu *vcpu, u64 *evmptr)
>> 
>> I prefer to rename evmptr to evmcs_ptr. I think it’s more readable and sufficiently short.
>> In addition, I think you should return either -1ull or assist_page.current_nested_vmcs.
>> i.e. Don’t return evmcs_ptr by pointer but instead as a return-value
>> and get rid of the bool.
> 
> Actually no, sorry, I'm having second thoughts here: in handle_vmclear()
> we don't care about the value of evmcs_ptr, we only want to check that
> enlightened vmentry bit is enabled in assist page. If we switch to
> checking evmcs_ptr against '-1', for example, we will make '-1' a magic
> value which is not in the TLFS. Windows may decide to use it for
> something else - and we will get a hard-to-debug bug again.

I’m not sure I understand.
You are worried that when guest have setup a valid assist-page and set enlighten_vmentry to true,
that assist_page.current_nested_vmcs can be -1ull and still be considered a valid eVMCS?
I don't think that's reasonable.

i.e. I thought about having this version of the method:

+u64 nested_enlightened_vmentry(struct kvm_vcpu *vcpu)
+{
+	struct hv_vp_assist_page assist_page;
+
+	if (unlikely(!kvm_hv_get_assist_page(vcpu, &assist_page)))
+		return -1ull;
+
+	if (unlikely(!assist_page.enlighten_vmentry))
+		return -1ull;
+
+	return assist_page.current_nested_vmcs;
+}
+

-Liran

> 
> If you still dislike nested_enlightened_vmentry() having the side effect
> of fetching evmcs_ptr I can get rid of it by splitting the function into
> two, however, it will be less efficient for
> nested_vmx_handle_enlightened_vmptrld(). Or we can just leave things as
> they are there and use the newly introduced function in handle_vmclear()
> only.
> 
> -- 
> Vitaly