linux-kernel - Re: [PATCHv2 08/12] KVM: TDX: Handle PAMT allocation in fault path

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Message-ID: <5c0b2e7acbfe59e8919cadfec1ab2503eec1a022.camel@intel.com>
Date: Thu, 10 Jul 2025 01:33:41 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "dave.hansen@...ux.intel.com"
	<dave.hansen@...ux.intel.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "bp@...en8.de" <bp@...en8.de>, "Huang,
 Kai" <kai.huang@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	"mingo@...hat.com" <mingo@...hat.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 08/12] KVM: TDX: Handle PAMT allocation in fault path

On Mon, 2025-06-09 at 22:13 +0300, Kirill A. Shutemov wrote:
>  int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
>  			      enum pg_level level, kvm_pfn_t pfn)
>  {
> +	struct kvm_vcpu *vcpu = kvm_get_running_vcpu();

This is unfortunate. In practice, all of the callers will be in a vCPU context,
but __tdp_mmu_set_spte_atomic() can be called for zap's which is why there is no
vCPU.

We don't want to split the tdp mmu calling code to introduce a variant that has
a vCPU. 

What about a big comment? Or checking for NULL and returning -EINVAL like
PG_LEVEL_4K below? I guess in this case a NULL pointer will be plenty loud. So
probably a comment is enough.

Hmm, the only reason we need the vCPU here is to get at the the per-vCPU pamt
page cache. This is also the reason for the strange callback scheme I was
complaining about in the other patch. It kind of seems like there are two
friction points in this series:
1. How to allocate dpamt pages
2. How to serialize the global DPAMT resource inside a read lock

I'd like to try to figure out a better solution for (1). (2) seems good. But I'm
still processing.

>  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
>  	struct page *page = pfn_to_page(pfn);
> +	int ret;
> +
> +	ret = tdx_pamt_get(page, level, tdx_alloc_pamt_page_atomic, vcpu);
> +	if (ret)
> +		return ret;