[Dropped non-kvm members from cc]
Marcelo Tosatti <mtosa...@redhat.com> wrote:
> VGABIOS mode constantly destroys and creates 0xa0000 slot, so
> performance is required for KVM_SET_MEM too (it can probably be fixed in
> qemu, but older qemu's must be supported).
Apart from srcu, I see some problems concerning slot creation/destruction:
heavy shadow flush (and extra write protection).
Look at __kvm_set_memory_region().
1. When we invalidate a memory slot, we call kvm_arch_flush_shadow() and
zap all shadow pages, not restricted to that slot.
/* From this point no new shadow pages pointing to a deleted
* memslot will be created.
*
* validation of sp->gfn happens in:
* - gfn_to_hva (kvm_read_guest, gfn_to_pfn)
* - kvm_is_visible_gfn (mmu_check_roots)
*/
kvm_arch_flush_shadow(kvm);
2. When we create(and shift?) a memory slot, we call kvm_arch_flush_shadow()
to clear all mmio sptes, again not restricted to that slot.
/*
* If the new memory slot is created, we need to clear all
* mmio sptes.
*/
if (npages && old.base_gfn != mem->guest_phys_addr >> PAGE_SHIFT)
kvm_arch_flush_shadow(kvm);
3. In addition to these, we do write-protect all pages in that slot in
kvm_arch_commit_memory_region() every time.
For 1: I made a patch to restrict the flush to that slot by using
sp->slot_bitmap.
(seems working here)
For 2: I think we can do the same: not 100% sure yet because I am still
struggling with the "mmio spte optimization" code. (really hacky ...)
For 3: I think doing both "write protection" and "shadow flush" is unnecessary.
BTW do we really need fast slot creation/destruction?
Takuya
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