linux-kernel - Re: [RFC, PATCH 08/12] KVM: x86/tdp_mmu: Add phys_prepare() and phys_cleanup() to kvm_x86

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Message-ID: <mgu7at7d3qy4h55bchxfmxj6yzqyi7gh4ieds4ecdvlv243frl@bzou376shiak>
Date: Fri, 23 May 2025 15:00:56 +0300
From: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "Zhao, Yan Y" <yan.y.zhao@...el.com>, 
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>, 
	"mingo@...hat.com" <mingo@...hat.com>, "tglx@...utronix.de" <tglx@...utronix.de>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>
Subject: Re: [RFC, PATCH 08/12] KVM: x86/tdp_mmu: Add phys_prepare() and
 phys_cleanup() to kvm_x86_ops

On Wed, May 14, 2025 at 12:00:17AM +0000, Huang, Kai wrote:
> On Mon, 2025-05-12 at 12:55 +0300, Kirill A. Shutemov wrote:
> > On Fri, May 09, 2025 at 09:25:58AM +0800, Yan Zhao wrote:
> > > On Thu, May 08, 2025 at 04:23:56PM +0300, Kirill A. Shutemov wrote:
> > > > On Tue, May 06, 2025 at 07:55:17PM +0800, Yan Zhao wrote:
> > > > > On Fri, May 02, 2025 at 04:08:24PM +0300, Kirill A. Shutemov wrote:
> > > > > > 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov