linux-kernel - Re: [PATCH RFC] KVM: TDX: Defer guest memory removal to decrease shutdown time

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Message-ID: <ful5rg4jmtxtpyf4apdgrcp3ohttqvwfdwbcrszf6h3jnlhlr5@pfkl6uvadwhu>
Date: Mon, 17 Mar 2025 10:13:50 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, kvm@...r.kernel.org, 
	rick.p.edgecombe@...el.com, kai.huang@...el.com, reinette.chatre@...el.com, 
	xiaoyao.li@...el.com, tony.lindgren@...ux.intel.com, binbin.wu@...ux.intel.com, 
	isaku.yamahata@...el.com, linux-kernel@...r.kernel.org, yan.y.zhao@...el.com, 
	chao.gao@...el.com
Subject: Re: [PATCH RFC] KVM: TDX: Defer guest memory removal to decrease
 shutdown time

On Thu, Mar 13, 2025 at 08:16:29PM +0200, Adrian Hunter wrote:
> @@ -3221,6 +3241,19 @@ int tdx_gmem_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn)
>  	return PG_LEVEL_4K;
>  }
>  
> +int tdx_gmem_defer_removal(struct kvm *kvm, struct inode *inode)
> +{
> +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> +
> +	if (kvm_tdx->nr_gmem_inodes >= TDX_MAX_GMEM_INODES)
> +		return 0;

We have graceful way to handle this, but should we pr_warn_once() or
something if we ever hit this limit?

Hm. It is also a bit odd that we need to wait until removal to add a link
to guest_memfd inode from struct kvm/kvm_tdx. Can we do it right away in
__kvm_gmem_create()?

Do I read correctly that inode->i_mapping->i_private_list only ever has
single entry of the gmem? Seems wasteful.

Maybe move it to i_private (I don't see flags being used anywhere) and
re-use the list_head to link all inodes of the struct kvm?

No need in the gmem_inodes array.
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov