linux-kernel - Re: [PATCH] KVM: x86/mmu: Embed direct bits into gpa for KVM_PRE_FAULT

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Message-ID: <aEt7bit5tTWRSHfz@yzhao56-desk.sh.intel.com>
Date: Fri, 13 Jun 2025 09:14:22 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>, "Chatre,
 Reinette" <reinette.chatre@...el.com>, "Hunter, Adrian"
	<adrian.hunter@...el.com>, "tony.lindgren@...ux.intel.com"
	<tony.lindgren@...ux.intel.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Embed direct bits into gpa for
 KVM_PRE_FAULT_MEMORY

On Fri, Jun 13, 2025 at 02:50:48AM +0800, Edgecombe, Rick P wrote:
> On Thu, 2025-06-12 at 15:19 +0800, Yan Zhao wrote:
> > > TDX isn't setting PFERR_WRITE_MASK or PFERR_PRESENT_MASK in the error_code
> > > passed into the fault handler. So page_fault_can_be_fast() should return
> > > false
> > > for that reason for private/mirror faults.
> > Hmm, TDX does set PFERR_WRITE_MASK in the error_code when fault->prefetch is
> > false (since exit_qual is set to EPT_VIOLATION_ACC_WRITE in
> > tdx_handle_ept_violation()).
> > 
> > PFERR_PRESENT_MASK is always unset.
> > 
> > page_fault_can_be_fast() does always return false for private mirror faults
> > though, due to the reason in
> > https://lore.kernel.org/kvm/aEp6pDQgbjsfrg2h@yzhao56-desk.sh.intel.com :)
> 
> Seems cleanup worthy to me, but not a bug. I think we should follow up,
> depending on the scope of Sean's cleanup.
Ok. There was an explicit disallowing of fast page fault for mirror [1].
Maybe we can add it back after Sean's cleanup.

[1] https://lore.kernel.org/kvm/af70ce8626cb7366d9b86a41c5d731f8ebd144ff.1708933498.git.isaku.yamahata@intel.com/