On Mon, 2025-05-19 at 16:32 +0800, Zhao, Yan Y wrote:
> > But in the above text you mentioned that, if doing so, because we choose to
> > ignore splitting request on read, returning 2M could result in *endless* EPT
> > violation.
> I don't get what you mean.
> What's the relationship between splitting and "returning 2M could result in
> *endless* EPT" ?
>
> > So to me it seems you choose a design that could bring performance gain for
> > certain non-Linux TDs when they follow a certain behaviour but otherwise could
> > result in endless EPT violation in KVM.
> Also don't understand here.
> Which design could result in endless EPT violation?
[Sorry somehow I didn't see your replies yesterday in my mailbox.]
You mentioned below in your coverletter:
(b) with shared kvm->mmu_lock, triggered by fault.
....
This series simply ignores the splitting request in the fault path to
avoid unnecessary bounces between levels. The vCPU that performs ACCEPT
at a lower level would finally figures out the page has been accepted
at a higher level by another vCPU.
... The worst outcome to ignore the resulting
splitting request is an endless EPT violation. This would not happen
for a Linux guest, which does not expect any #VE.
So to me, IIUC, this means:
- this series choose to ignore splitting request when read ..
- the worse outcome to ignore the resulting splitting request is an endless
EPT violation..
And this happens exactly in below case:
1) Guest touches a 4K page
2) KVM AUGs 2M page
3) Guest re-accesses that 4K page, and receives #VE
4) Guest ACCEPTs that 4K page, this triggers EPT violation
IIUC, you choose to ignore splitting large page in step 4) (am I right???).
Then if guest always ACCEPTs page at 4K level, then KVM will have *endless EPT
violation*.
So, is this the "worst outcome to ignore the resulting splitting request" that
you mentioned in your changelog?
If it is, then why is it OK?
It is OK *ONLY* when "guest always ACCEPTs 4K page" is a buggy behaviour of the
guest itself (which KVM is not responsible for). I.e., the guest is always
supposed to find the page size that KVM has AUGed upon receiving the #VE (does
the #VE contain such information?) and then do ACCEPT at that page level.
Otherwise, if it's a legal behaviour for the guest to always ACCEPT at 4K level,
then I don't think it's OK to have endless EPT violation in KVM.
