linux-kernel - Re: [PATCH v12 11/29] KVM: SEV: Add KVM_SEV_SNP_LAUNCH

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Date: Wed, 3 Apr 2024 14:51:59 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
Cc: Michael Roth <michael.roth@....com>, kvm@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org, 
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, 
	jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com, ardb@...nel.org, 
	seanjc@...gle.com, vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org, 
	dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com, 
	peterz@...radead.org, srinivas.pandruvada@...ux.intel.com, 
	rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de, 
	vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com, tony.luck@...el.com, 
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	jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com, 
	pankaj.gupta@....com, liam.merwick@...cle.com, 
	Brijesh Singh <brijesh.singh@....com>, Xu Yilun <yilun.xu@...ux.intel.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v12 11/29] KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command

On Wed, Apr 3, 2024 at 12:58 AM Isaku Yamahata <isaku.yamahata@...el.com> wrote:
> I think TDX can use it with slight change. Pass vcpu instead of KVM, page pin
> down and mmu_lock.  TDX requires non-leaf Secure page tables to be populated
> before adding a leaf.  Maybe with the assumption that vcpu doesn't run, GFN->PFN
> relation is stable so that mmu_lock isn't needed? What about punch hole?
>
> The flow would be something like as follows.
>
> - lock slots_lock
>
> - kvm_gmem_populate(vcpu)
>   - pin down source page instead of do_memcopy.

Both pinning the source page and the memcpy can be done in the
callback.  I think the right thing to do is:

1) eliminate do_memcpy, letting AMD code taking care of
   copy_from_user.

2) pass to the callback only gfn/pfn/src, where src is computed as

    args->src ? args->src + i * PAGE_SIZE : NULL

If another architecture/vendor needs do_memcpy, they can add
something like kvm_gmem_populate_copy.

>   - get pfn with __kvm_gmem_get_pfn()
>   - read lock mmu_lock
>   - in the post_populate callback
>     - lookup tdp mmu page table to check if the table is populated.
>       lookup only version of kvm_tdp_mmu_map().
>       We need vcpu instead of kvm.

Passing vcpu can be done using the opaque callback argument to
kvm_gmem_populate.

Likewise, the mmu_lock can be taken by the TDX post_populate
callback.

Paolo

>     - TDH_MEM_PAGE_ADD
>   - read unlock mmu_lock
>
> - unlock slots_lock
>
> Thanks,
>
> > With that model, the potential for using kvm_gmem_populate() seemed
> > plausible to I was trying to make it immediately usable for that
> > purpose. But maybe the TDX folks can confirm whether this would be
> > usable for them or not. (kvm_gmem_populate was introduced here[2] for
> > reference/background)
> >
> > -Mike
> >
> > [1] https://lore.kernel.org/kvm/20240319155349.GE1645738@ls.amr.corp.intel.com/T/#m8580d8e39476be565534d6ff5f5afa295fe8d4f7
> > [2] https://lore.kernel.org/kvm/20240329212444.395559-3-michael.roth@amd.com/T/#m3aeba660fcc991602820d3703b1265722b871025)