On 05/30/2011 11:05 AM, Ingo Molnar wrote:
* Avi Kivity<avi@xxxxxxxxxx> wrote: > On 05/30/2011 10:40 AM, Ingo Molnar wrote: > >* Yang, Wei Y<wei.y.yang@xxxxxxxxx> 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.
It's not a good idea. First, the guest will see cr4.smep where it hasn't set it before, which may confuse it. Second, the guest may rewrite cr4.smep, clearing it, giving a false sense of security.
> [...] 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.
Is there any reason not to do it in a hidden way in kvm? Why must we play tricks?
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