linux-kernel - Re: [PATCH 4/4] KVM: nVMX: initialize more non-shadowed fields in prepare_vmcs02

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Message-ID: <CANRm+CzaAQThz5u5fdNFrw0uxR5Geq0DgCHjAzeVOOpW9gNEQQ@mail.gmail.com>
Date:   Thu, 28 Dec 2017 10:07:10 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH 4/4] KVM: nVMX: initialize more non-shadowed fields in prepare_vmcs02_full

2017-12-27 17:54 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
> On 25/12/2017 04:09, Wanpeng Li wrote:
>> 2017-12-21 20:43 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>>> These fields are also simple copies of the data in the vmcs12 struct.
>>> For some of them, prepare_vmcs02 was skipping the copy when the field
>>> was unused.  In prepare_vmcs02_full, we copy them always as long as the
>>> field exists on the host, because the corresponding execution control
>>> might be one of the shadowed fields.
>>
>> Why we don't need to copy them always before the patchset?
>
> Before these patches, we only copy them if the corresponding processor
> control is enabled.  For example, we only copy the EOI exit bitmap if
> APICv is enabled by L1.  Here we could have
>
>    write to EOI exit bitmap
>    vmlaunch (calls prepare_vmcs02_full)
>    enable APICv (but EOI exit bitmap fields are clean)
>    vmresume (doesn't call prepare_vmcs02_full)
>
> The vmresume doesn't call prepare_vmcs02_full, so the EOI exit bitmap
> must be copied every time prepare_vmcs02_full runs.

I see, what about my question to patch 2/4?  Writing to vmcs fields
always causes vmexit if enable_shadow_vmcs == false and
vmx->nested.dirty_vmcs12 is false for "shadow vmcs fields", this can
result in prepare_vmcs02_full() is not called even if some processor
controls are modified.

Regards,
Wanpeng Li