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Re: Questions about the real mode in kvm/qemu
From: |
Paolo Bonzini |
Subject: |
Re: Questions about the real mode in kvm/qemu |
Date: |
Thu, 26 Sep 2019 11:33:05 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 |
On 26/09/19 11:24, Maxim Levitsky wrote:
> On Thu, 2019-09-26 at 11:18 +0200, Paolo Bonzini wrote:
>> On 26/09/19 10:59, Maxim Levitsky wrote:
>>> If you mean to ask if there is a way to let guest access use no
>>> paging at all, that is access host physical addresses directly, then
>>> indeed there is no way, since regular non 'unrestricted guest' mode
>>> required both protected mode and paging, and 'unrestricted guest'
>>> requires EPT. Academically speaking it is of course possible to
>>> create paging tables that are 1:1...
>>
>> Not so academically, it's exactly what KVM does.
> You mean KVM uses 1:1 EPT pages and no guest paging,
> to allow guest to access host physical address space?
No, it uses the usual HVA->GPA EPT pages and 1:1 GPA->GVA pages when EPT
is enabled and guest CR0.PG=0. This lets KVM work around the CR0.PG=1
requirement when unrestricted guest mode.
Thinking more about it, I suppose that saves memory (the same EPT page
tables can now be used independent of guest CR0.PG), at the cost of
making TLB misses a little slower.
Thanks,
Paolo
>> However, indeed it
>> would also be possible to switch out of EPT mode when CR0.PG=0. I'm not
>> sure why it was done this way, maybe when the code was written it was
>> simpler to use the identity map.
>>
>> Let's see if Avi is listening... :)
>>
>> Paolo
>
> Here a quote from the PRM:
>
> "The first processors to support VMX operation require CR0.PE and CR0.PG to
> be 1 in VMX operation (see Section
> 23.8). This restriction implies that guest software cannot be run in unpaged
> protected mode or in real-address
> mode. Later processors support a VM-execution control called “unrestricted
> guest”. 1 If this control is 1, CR0.PE and
> CR0.PG may be 0 in VMX non-root operation. Such processors allow guest
> software to run in unpaged protected
> mode or in real-address mode. The following items describe the behavior of
> such software:"
> ...
>
> "As noted in Section 26.2.1.1, the “enable EPT” VM-execution control must be
> 1 if the “unrestricted guest” VM-execution control is 1."
>
>
> Best regards,
> Maxim Levitsky
>
- Questions about the real mode in kvm/qemu, Li Qiang, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Maxim Levitsky, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Li Qiang, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Maxim Levitsky, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Paolo Bonzini, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Maxim Levitsky, 2019/09/26
- Re: Questions about the real mode in kvm/qemu,
Paolo Bonzini <=
- Re: Questions about the real mode in kvm/qemu, Maxim Levitsky, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Paolo Bonzini, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Maxim Levitsky, 2019/09/26
- Re: Questions about the real mode in kvm/qemu, Avi Kivity, 2019/09/28
- Re: Questions about the real mode in kvm/qemu, Li Qiang, 2019/09/29
Re: Questions about the real mode in kvm/qemu, Paolo Bonzini, 2019/09/26