* Avi Kivity <[email protected]> wrote:
> On 05/30/2011 10:40 AM, Ingo Molnar wrote:
> >* Yang, Wei Y<[email protected]> wrote:
> >
> >> This patch removes SMEP bit from CR4_RESERVED_BITS.
> >
> >I'm wondering, what is the best-practice way for tools/kvm/ to set
> >SMEP for the guest kernel automatically, even if the guest kernel
> >itsef has not requested SMEP?
> >
> > The portion i'm worried about are old KVM versions that have the
> > SMEP bit in CR4_RESERVED_BITS and reject it. So we cannot just
> > unilaterally add SMEP to every cr4 write of the guest.
>
> tools/kvm doesn't see cr4 writes at all. [...]
I feared small complications like that! :-)
We can definitely use KVM_GET_SREGS, fiddle the SMEP bit in
kvm_regs.cr4 and call KVM_SET_SREGS, once the fine patch above goes
upstream.
> [...] The only way to do this is in kvm itself.
>
> > Is there a way to query whether the host KVM version supports
> > SMEP setting in cr4?
> >
>
> KVM_GET_SUPPORTED_CPUID (it returns whether both the host cpu and
> kvm support smep; if one of them doesn't, you'll see smep
> disabled).
That looks useful.
So the way to go appears to be to do a GET_SREGS/SET_SREGS sequence
to enable SMEP in the guest, some time after it has booted and has
enabled paging.
I'm wondering whether there's a suitable place to do that, when we
are more or less guaranteed to exit the VM for some other reason -
such as the first MMIO done with paging enabled?
This solution means that we'll slow down pre-paging MMIOs with a
GET_SREGS call, but that's ok, they are rare and the pre-paging
bootup phase is very short.
So the only worry would be where the guest sets cr4 itself - and
since it does not know about SMEP it will probably disable it. Guest
suspend/resume is one such place ...
Another option would be to try to set the SMEP bit *before* we enable
paging. In theory this should not confuse a Linux guest - and while i
have not tested it i *think* we let it survive in the
saved_cr4_features shadow variable. That would make guest
suspend/resume work out of box as well.
Thanks,
Ingo
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