Re: [PATCH v7 000/102] KVM TDX basic feature support
From: Xiaoyao Li
Date: Thu Jul 14 2022 - 00:10:09 EST
On 7/14/2022 9:03 AM, Sean Christopherson wrote:
On Mon, Jun 27, 2022, isaku.yamahata@xxxxxxxxx wrote:
From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
KVM TDX basic feature support
Hello. This is v7 the patch series vof KVM TDX support.
This is based on v5.19-rc1 + kvm/queue branch + TDX HOST patch series.
The tree can be found at https://github.com/intel/tdx/tree/kvm-upstream
How to run/test: It's describe at https://github.com/intel/tdx/wiki/TDX-KVM
Major changes from v6:
- rebased to v5.19 base
TODO:
- integrate fd-based guest memory. As the discussion is still on-going, I
intentionally dropped fd-based guest memory support yet. The integration can
be found at https://github.com/intel/tdx/tree/kvm-upstream-workaround.
- 2M large page support. It's work-in-progress.
For large page support, there are several design choices. Here is the design options.
Any thoughts/feedback?
Apologies, I didn't read beyond the intro paragraph. In case something like this
comes up again, it's probably best to send a standalone email tagged RFC, I doubt
I'm the only one that missed this embedded RFC.
KVM MMU Large page support for TDX
...
* options to track private or shared
At each page size (4KB, 2MB, and 1GB), track private, shared, or mixed (2MB and
1GB case). For 4KB each page, 1 bit per page is needed. private or shared. For
large pages (2MB and 1GB), 2 bits per large page is needed. (private, shared, or
mixed). When resolving KVM page fault, we don't want to check the lower-size
pages to check if the given GPA can be a large for performance. On MapGPA check
it instead.
Option A). enhance kvm_arch_memory_slot
enum kvm_page_type {
KVM_PAGE_TYPE_INVALID,
KVM_PAGE_TYPE_SHARED,
KVM_PAGE_TYPE_PRIVATE,
KVM_PAGE_TYPE_MIXED,
};
struct kvm_page_attr {
enum kvm_page_type type;
};
struct kvm_arch_memory_slot {
+ struct kvm_page_attr *page_attr[KVM_NR_PAGE_SIZES];
Option B). steal one more bit SPTE_MIXED_MASK in addition to SPTE_SHARED_MASK
If !SPTE_MIXED_MASK, it can be large page.
I don't think this is a good option, since it requires all the mappings
exist all the time both in shared spte tree and private spte tree.
Option C). use SPTE_SHARED_MASK and kvm_mmu_page::mixed bitmap
kvm_mmu_page::mixed bitmap of 1GB, root indicates mixed for 2MB, 1GB.
* comparison
A).
+ straightforward to implement
+ SPTE_SHARED_MASK isn't needed
- memory overhead compared to B). or C).
- more memory reference on KVM page fault
B).
+ simpler than C) (complex than A)?)
+ efficient on KVM page fault. (only SPTE reference)
+ low memory overhead
- Waste precious SPTE bits.
C).
+ efficient on KVM page fault. (only SPTE reference)
+ low memory overhead
- complicates MapGPA
- scattered data structure
Option D). track shared regions in an Xarray, update kvm_arch_memory_slot.lpage_info
on insertion/removal to (dis)allow hugepages as needed.
UPM v7[1] introduces "struct xarray mem_attr_array" to track the
shared/private attr of a range.
So in kvm_vm_ioctl_set_encrypted_region() it needs to
- increase the lpage_info counter when a 2m/1g range changed from
identical to mixed, and
- decrease the counter when mixed -> identical
[1]:
https://lore.kernel.org/all/20220706082016.2603916-12-chao.p.peng@xxxxxxxxxxxxxxx/
+ efficient on KVM page fault (no new lookups)
+ zero memory overhead (assuming KVM has to eat the cost of the Xarray anyways)
+ straightforward to implement
+ can (and should) be merged as part of the UPM series
I believe xa_for_each_range() can be used to see if a given 2mb/1gb range is
completely covered (fully shared) or not covered at all (fully private), but I'm
not 100% certain that xa_for_each_range() works the way I think it does.