linux-kernel - Re: [PATCH V3 1/2] x86/tdx: Eliminate duplicate code in tdx_clear

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Message-ID: <d0a42336-89ba-4317-9e0b-7d51a09b1567@intel.com>
Date: Tue, 22 Jul 2025 17:48:22 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Dave Hansen <dave.hansen@...ux.intel.com>, <pbonzini@...hat.com>,
	<vannapurve@...gle.com>, Tony Luck <tony.luck@...el.com>, Borislav Petkov
	<bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
	<mingo@...hat.com>, <x86@...nel.org>, H Peter Anvin <hpa@...or.com>,
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<rick.p.edgecombe@...el.com>, <kas@...nel.org>, <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>, <yan.y.zhao@...el.com>, <chao.gao@...el.com>
Subject: Re: [PATCH V3 1/2] x86/tdx: Eliminate duplicate code in
 tdx_clear_page()

On 22/07/2025 17:11, Sean Christopherson wrote:
> On Tue, Jul 22, 2025, Adrian Hunter wrote:
>> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
>> index 7ddef3a69866..f66328404724 100644
>> --- a/arch/x86/include/asm/tdx.h
>> +++ b/arch/x86/include/asm/tdx.h
>> @@ -131,6 +131,8 @@ int tdx_guest_keyid_alloc(void);
>>  u32 tdx_get_nr_guest_keyids(void);
>>  void tdx_guest_keyid_free(unsigned int keyid);
>>  
>> +void tdx_quirk_reset_paddr(unsigned long base, unsigned long size);
>> +
>>  struct tdx_td {
>>  	/* TD root structure: */
>>  	struct page *tdr_page;
>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
>> index 573d6f7d1694..1b549de6da06 100644
>> --- a/arch/x86/kvm/vmx/tdx.c
>> +++ b/arch/x86/kvm/vmx/tdx.c
>> @@ -283,25 +283,6 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
>>  	vcpu->cpu = -1;
>>  }
>>  
>> -static void tdx_clear_page(struct page *page)
>> -{
>> -	const void *zero_page = (const void *) page_to_virt(ZERO_PAGE(0));
>> -	void *dest = page_to_virt(page);
>> -	unsigned long i;
>> -
>> -	/*
>> -	 * The page could have been poisoned.  MOVDIR64B also clears
>> -	 * the poison bit so the kernel can safely use the page again.
>> -	 */
>> -	for (i = 0; i < PAGE_SIZE; i += 64)
>> -		movdir64b(dest + i, zero_page);
>> -	/*
>> -	 * MOVDIR64B store uses WC buffer.  Prevent following memory reads
>> -	 * from seeing potentially poisoned cache.
>> -	 */
>> -	__mb();
>> -}
>> -
>>  static void tdx_no_vcpus_enter_start(struct kvm *kvm)
>>  {
>>  	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
>> @@ -347,7 +328,7 @@ static int tdx_reclaim_page(struct page *page)
>>  
>>  	r = __tdx_reclaim_page(page);
>>  	if (!r)
>> -		tdx_clear_page(page);
>> +		tdx_quirk_reset_paddr(page_to_phys(page), PAGE_SIZE);
> 
> This is silly.  Literally every use in KVM is on a struct page.  I agree with
> Dave that having a wrapper with a completely unrelated name is confusing, but
> that's a naming problem, not a code problem.
> 
> And FWIW, I find tdx_quirk_reset_paddr() confusing, because it reads like it's
> resetting the address itself.  But if KVM only ever uses tdx_quirk_reset_page(),
> I don't care what you call the inner helper.

As you say, Dave's second option was:

	"The alternative would be to retain a function that keeps the 'struct
	page' as an argument. Something like:

		tdx_quirk_reset_paddr(unsigned long base, unsigned long size)
	and
		tdx_quirk_reset_page(struct page *page)"

So I will do that for V4 unless there are further comments.