[Xen-users] Guest PGE with VPID tagged TLB

[Tamas Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

Hello all,
I've been experimenting with the TLB in HVM guests where a PTE is accessed which has the global bit enabled. I checked that CR4.PGE is also enabled in the guest and that the guest itself doesn't flush the TLB for global pages (no flipping of the PGE bit is done by the guest). However, the TLB is never hit and the guest always performs a pagetable lookup.

I checked how asid/vpid is generated by Xen, which is essentially every time the domain performs a mov cr3/cr4. This results in missing the TLB entries for the global PTEs because the tags don't match. If this is indeed the case, this means that PGE is effectively unusable for all guests running on Xen. The point of the PGE was that it would retain the TLB entries during a context switch for the global PDEs/PTEs. Now, the TLB entries are retained with Xen but effectively inaccessible because the tags don't match. I haven't found documentation of this behavior so it would be nice to verify that this is indeed the case and maybe note this somewhere in the documentation/wiki. So, is my understanding on what Xen is doing correct?

Thanks,

Tamas

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