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

From: Liran Alon
Date: Tue Jun 25 2019 - 07:02:26 EST

> On 25 Jun 2019, at 11:51, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> Liran Alon <liran.alon@xxxxxxxxxx> writes:
>>> On 24 Jun 2019, at 16:30, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>>> +bool nested_enlightened_vmentry(struct kvm_vcpu *vcpu, u64 *evmptr)
>> I prefer to rename evmptr to evmcs_ptr. I think itâs more readable and sufficiently short.
>> In addition, I think you should return either -1ull or assist_page.current_nested_vmcs.
>> i.e. Donât 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.

Iâm 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;


> 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