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: <aB1ZplDCPkDCkhQr@yzhao56-desk.sh.intel.com>
Date: Fri, 9 May 2025 09:25:58 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC: <pbonzini@...hat.com>, <seanjc@...gle.com>, <rick.p.edgecombe@...el.com>,
	<isaku.yamahata@...el.com>, <kai.huang@...el.com>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<kvm@...r.kernel.org>, <x86@...nel.org>, <linux-coco@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC, PATCH 08/12] KVM: x86/tdp_mmu: Add phys_prepare() and
 phys_cleanup() to kvm_x86_ops

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? 


[1] https://lore.kernel.org/all/20250424030926.554-1-yan.y.zhao@intel.com/