Re: [RFC PATCH 1/2] kvm/x86: intercept guest changes to X86_CR4_LA57

[Lai Jiangshan]

On Tue, Sep 29, 2020 at 12:24 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Mon, Sep 28, 2020 at 04:30:46PM +0800, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> >
> > When shadowpaping is enabled, guest should not be allowed
> > to toggle X86_CR4_LA57. And X86_CR4_LA57 is a rarely changed
> > bit, so we can just intercept all the attempts to toggle it
> > no matter shadowpaping is in used or not.
> >
> > Fixes: fd8cb433734ee ("KVM: MMU: Expose the LA57 feature to VM.")
> > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Cc: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> >   No test to toggle X86_CR4_LA57 in guest since I can't access to
> >   any CPU supports it. Maybe it is not a real problem.
>


Hello

Thanks for reviewing.

> LA57 doesn't need to be intercepted.  It can't be toggled in 64-bit mode
> (causes a #GP), and it's ignored in 32-bit mode.  That means LA57 can only
> take effect when 64-bit mode is enabled, at which time KVM will update its
> MMU context accordingly.
>

Oh, I missed that part which is so obvious that the patch
seems impertinent.

But X86_CR4_LA57 is so fundamental that it makes me afraid to
give it over to guests. And it is rarely changed too. At least,
there is no better reason to give it to the guest than
intercepting it.

There might be another reason that this patch is still needed with
an updated changelog.

When a user (via VMM such as qemu) launches a VM with LA57 disabled
in its cpuid on a LA57 enabled host. The hypervisor, IMO, needs to
intercept guest's changes to X86_CR4_LA57 even when the guest is still
in the non-paging mode. Otherwise the hypervisor failed to detective
such combination when the guest changes paging mode later.

Anyway, maybe it is still not a real problem.

Thanks
Lai