linux-kernel - Re: [PATCH] KVM-Use value reading from MSR when construct the eptp in VMX mode

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Message-ID: <CA+U0gVh_dWztJ4CQzbNTUKFmew75RE4kc=qrikdFm6hs3S2J0Q@mail.gmail.com>
Date:	Tue, 26 Aug 2014 10:34:18 +0800
From:	Dennis Chen <kernel.org.gnu@...il.com>
To:	Gleb Natapov <gleb@...nel.org>
Cc:	kvm <kvm@...r.kernel.org>, linux-kernel@...r.kernel.org,
	pbonzini@...hat.com, "wanpeng.li" <wanpeng.li@...ux.intel.com>
Subject: Re: [PATCH] KVM-Use value reading from MSR when construct the eptp in
 VMX mode

On Mon, Aug 25, 2014 at 10:04 PM, Gleb Natapov <gleb@...nel.org> wrote:
> On Mon, Aug 25, 2014 at 11:16:34AM +0800, Dennis Chen wrote:
>> On Sun, Aug 24, 2014 at 5:38 PM, Gleb Natapov <gleb@...nel.org> wrote:
>> > On Sun, Aug 24, 2014 at 11:54:32AM +0800, Dennis Chen wrote:
>> >> This patch is used to construct the eptp in vmx mode with values
>> >> readed from MSR according to the intel x86 software developer's
>> >> manual.
>> >>
>> >>  static u64 construct_eptp(unsigned long root_hpa)
>> >>  {
>> >> -    u64 eptp;
>> >> +    u64 eptp, pwl;
>> >> +
>> >> +    if (cpu_has_vmx_ept_4levels())
>> >> +        pwl = VMX_EPT_DEFAULT_GAW << VMX_EPT_GAW_EPTP_SHIFT;
>> >> +    else {
>> >> +        WARN(1, "Unsupported page-walk length of 4.\n");
>> > Page-walk length of 4 is the only one that is supported.
>> >
>> Since there is a bit 6 in IA32_VMX_EPT_VPID_CAP MSR indicating the
>> support for the page-walk length, I think sanity check is necessary.
>> But I just checked the code, it's already done in the hardware_setup()
>> function which will disable ept feature if the page-wake length is not
>> 4. Gleb, any comments for the memory type check part?
> Looks fine, but are there CPUs out there that do not support WB for eptp? Since
> there was no bug reports about it I assume no.

Hmm, currently I can't find a x86 processor that don't support WB for
eptp, also there is no relevant bug reported.
I just read the intel SDM 24.6.11: SW should read the VMX capability
MSR_IA32_VMX_EPT_VPID_CAP to determine
what EPT MT are supported. But looks like this is not a big concern in
the community, so let's go back this thread if
we encounter one unfornately in the future. Thanks for the comments.

>
> --
>                         Gleb.



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