Re: [Xen-devel] [PATCHv3 1/2] x86/ept: invalidate guest physical mappings on VMENTER

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 04/12/15 11:00, George Dunlap wrote:
> On 03/12/15 16:42, David Vrabel wrote:
>> If a guest allocates a page and the tlbflush_timestamp on the page
>> indicates that a TLB flush of the previous owner is required, only the
>> linear and combined mappings are invalidated.  The guest-physical
>> mappings are not invalidated.
>>
>> This is currently safe because the EPT code ensures that the
>> guest-physical and combined mappings are invalidated /before/ the page
>> is freed.  However, this prevents us from deferring the EPT invalidate
>> until after the page is freed (e.g., to defer the invalidate until the
>> p2m locks are released).
>>
>> The TLB flush that may be done after allocating page already causes
>> the original guest to VMEXIT, thus on VMENTER we can do an INVEPT if
>> one is still pending.
>>
>> ept_sync_domain() now marks all PCPUs as needing to be invalidated,
>> including PCPUs that the domain has not run on.  We still only IPI
>> those PCPUs that are active so this does not result in any more[1]
>> INVEPT calls.
>>
>> We do not attempt to track when PCPUs may have cached translations
>> because the only safe way to clear this per-CPU state if immediately
>> after an invalidate the PCPU is not active (i.e., the PCPU is not in
>> d->domain_dirty_cpumask).  Since we only invalidate on VMENTER or by
>> IPIing active PCPUs this can never happen.
>>
>> [1] There is one unnecessary INVEPT when the domain runs on a PCPU for
>>     the very first time.  But this is: a) only 1 additional invalidate
>>     per PCPU for the lifetime of the domain; and b) we can avoid it
>>     with a subsequent change.
>>
>> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> This looks like a definite improvement.
> 
> One thing you missed is that in ept_p2m_init(), it calls
> __ept_sync_domain() on all cpus; but because the "invalidate" mask is
> clear at that point, nothing will actually happen.

Good point.  I'd missed this because I'd planned to replace this initial
invalidate by initializing ept->invalidate to all ones (as I alluded to
in the [1] footnote).

> Part of this I think comes as a result from inverting what the mask
> means.  Before this patch, "not synced" is the default, and therefore
> you need to invalidate unless someone has set the bit saying you don't
> have to.  After this, "don't invalidate" is the default and you do
> nothing unless someone has set a bit saying you do have to.
> 
> I'd think prefer it if you left the mask as "synced_mask"; then you can
> actually take that on_each_cpu() out of the ept_p2m_init entirely.
> (Probably wants a comment pointing that out.)

I changed its name because it's old use as synced_mask (i.e., the set of
CPUs needing to be notified of required invalidation) did not match its
name.  I rather not keep the name and invert its use.

David

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