linux-kernel - Re: [RFC PATCH v2 11/23] KVM: x86: Reject splitting huge pages under shared mmu

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Message-ID: <deeb7ab5-c9e6-44c2-9136-f8e5abe31210@linux.intel.com>
Date: Wed, 3 Sep 2025 11:30:33 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, x86@...nel.org, rick.p.edgecombe@...el.com,
 dave.hansen@...el.com, kas@...nel.org, tabba@...gle.com,
 ackerleytng@...gle.com, quic_eberman@...cinc.com, michael.roth@....com,
 david@...hat.com, vannapurve@...gle.com, vbabka@...e.cz,
 thomas.lendacky@....com, pgonda@...gle.com, zhiquan1.li@...el.com,
 fan.du@...el.com, jun.miao@...el.com, ira.weiny@...el.com,
 isaku.yamahata@...el.com, xiaoyao.li@...el.com, chao.p.peng@...el.com
Subject: Re: [RFC PATCH v2 11/23] KVM: x86: Reject splitting huge pages under
 shared mmu_lock for mirror root



On 8/7/2025 5:43 PM, Yan Zhao wrote:
> While removing the KVM_BUG_ON() for the mirror root before invoking
> tdp_mmu_split_huge_page() in the fault path, update the hook
> split_external_spt to pass in shared mmu_lock info and invoke the hook in
> set_external_spte_present() on splitting is detected. Reject the splitting
> in TDX if the splitting is under shared mmu_lock.
>
> TDX requires different handling for splitting under shared or exclusive
> mmu_lock.
>
> Under a shared mmu_lock, TDX cannot kick off all vCPUs to avoid BUSY error
> from tdh_mem_page_demote(). As the current TDX module requires
> tdh_mem_range_block() to be invoked before each tdh_mem_page_demote(), if a
> BUSY error occurs, TDX must call tdh_mem_range_unblock() before returning
> the error to the KVM MMU core to roll back the old SPTE and retry. However,
> tdh_mem_range_unblock() may also fail due to contention.
>
> Reject splitting huge pages under shared mmu_lock for mirror root in TDX
> rather than KVM_BUG_ON() in KVM MMU core to allow for future real
> implementation of demote under shared mmu_lock once non-blocking demote is
> available.

Prefer "blockless" used in the cover letter to non-blocking.

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