On Fri, May 23, 2025 at 03:00:56PM +0300, kirill.shutemov@linux.intel.com wrote:

On Wed, May 14, 2025 at 12:00:17AM +0000, Huang, Kai wrote:

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On Mon, 2025-05-12 at 12:55 +0300, Kirill A. Shutemov wrote:

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On Fri, May 09, 2025 at 09:25:58AM +0800, Yan Zhao wrote:

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On Thu, May 08, 2025 at 04:23:56PM +0300, Kirill A. Shutemov wrote:

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On Tue, May 06, 2025 at 07:55:17PM +0800, Yan Zhao wrote:

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On Fri, May 02, 2025 at 04:08:24PM +0300, Kirill A. Shutemov wrote:

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The functions kvm_x86_ops::link_external_spt() and
kvm_x86_ops::set_external_spte() are used to assign new memory to a VM.
When using TDX with Dynamic PAMT enabled, the assigned memory must be
covered by PAMT.

The new function kvm_x86_ops::phys_prepare() is called before
link_external_spt() and set_external_spte() to ensure that the memory is
ready to be assigned to the virtual machine. In the case of TDX, it
makes sure that the memory is covered by PAMT.

kvm_x86_ops::phys_prepare() is called in a context where struct kvm_vcpu
is available, allowing the implementation to allocate memory from a
per-VCPU pool.

Why not invoke phys_prepare() and phys_cleanup() in set_external_spte_present()?
Or in tdx_sept_set_private_spte()/tdx_sept_link_private_spt()?

Because the memory pool we allocated from is per-vcpu and we lost access
to vcpu by then. And not all callers provide vcpu.

Maybe we can get vcpu via kvm_get_running_vcpu(), as in [1].
Then for callers not providing vcpu (where vcpu is NULL), we can use per-KVM
cache? 

Hm. I was not aware of kvm_get_running_vcpu(). Will play with it, thanks.

I am not sure why per-vcpu cache matters.

For non-leaf SEPT pages, AFAICT the "vcpu->arch.mmu_external_spt_cache" is just
an empty cache, and eventually __get_free_page() is used to allocate in:
                                                                                            
  sp->external_spt = 
	kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_external_spt_cache);

So why not we actually create a kmem_cache for it with an actual 'ctor', and we
can call tdx_alloc_page() in that.  This makes sure when the "external_spt" is
allocated, the underneath PAMT entry is there.

I looked closer to this and while it is good idea, but ctor in kmem_cache
cannot fail which makes this approach not viable.

I guess we can a constructor directly into struct kvm_mmu_memory_cache.
Let me play with this.

I failed to make it work.

We need to have destructor paired with the constructor that would do
PAMT-aware freeing. And redirect all free paths to it. It requires
substantial rework. I don't think it worth the effort.

Will do manual PAMT management for SPT in TDX code.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov