linux-kernel - Re: [PATCH v2] KVM: x86: Modify struct kvm_nested

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Date:   Wed, 19 Jun 2019 12:45:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Modify struct kvm_nested_state to have
 explicit fields for data

On 19/06/19 00:36, Liran Alon wrote:
> 
> 
>> On 18 Jun 2019, at 19:24, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> From: Liran Alon <liran.alon@...cle.com>
>>
>> Improve the KVM_{GET,SET}_NESTED_STATE structs by detailing the format
>> of VMX nested state data in a struct.
>>
>> In order to avoid changing the ioctl values of
>> KVM_{GET,SET}_NESTED_STATE, there is a need to preserve
>> sizeof(struct kvm_nested_state). This is done by defining the data
>> struct as "data.vmx[0]". It was the most elegant way I found to
>> preserve struct size while still keeping struct readable and easy to
>> maintain. It does have a misfortunate side-effect that now it has to be
>> accessed as "data.vmx[0]" rather than just "data.vmx".
>>
>> Because we are already modifying these structs, I also modified the
>> following:
>> * Define the "format" field values as macros.
>> * Rename vmcs_pa to vmcs12_pa for better readability.
>>
>> Signed-off-by: Liran Alon <liran.alon@...cle.com>
>> [Remove SVM stubs, add KVM_STATE_NESTED_VMX_VMCS12_SIZE. - Paolo]
> 
> 1) Why should we remove SVM stubs? I think it makes the interface intention more clear.
> Do you see any disadvantage of having them?

In its current state I think it would not require any state apart from
the global flags, because MSRs can be extracted independent of
KVM_GET_NESTED_STATE; this may change as things are cleaned up, but if
that remains the case there would be no need for SVM structs at all.

> 2) What is the advantage of defining a separate KVM_STATE_NESTED_VMX_VMCS12_SIZE
> rather than just moving VMCS12_SIZE to userspace header?

It's just for namespace cleanliness.  I'm keeping VMCS12_SIZE for the
arch/x86/kvm/vmx/ code because it's shorter and we're used to it, but
userspace headers should use a more specific name.

Paolo