linux-kernel - Re: [RFC PATCH 02/21] x86/virt/tdx: Enhance tdh_mem_page

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Message-ID: <9376b309-8561-4fcc-9e71-3bd03fd8f9d0@suse.com>
Date: Thu, 19 Jun 2025 12:26:07 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Yan Zhao <yan.y.zhao@...el.com>,
 "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>,
 "Shutemov, Kirill" <kirill.shutemov@...el.com>,
 "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>,
 "Li, Xiaoyao" <xiaoyao.li@...el.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "Hansen, Dave" <dave.hansen@...el.com>, "david@...hat.com"
 <david@...hat.com>, "thomas.lendacky@....com" <thomas.lendacky@....com>,
 "tabba@...gle.com" <tabba@...gle.com>, "Li, Zhiquan1"
 <zhiquan1.li@...el.com>, "Du, Fan" <fan.du@...el.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "michael.roth@....com" <michael.roth@....com>,
 "Weiny, Ira" <ira.weiny@...el.com>, "vbabka@...e.cz" <vbabka@...e.cz>,
 "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
 "ackerleytng@...gle.com" <ackerleytng@...gle.com>,
 "Yamahata, Isaku" <isaku.yamahata@...el.com>,
 "Peng, Chao P" <chao.p.peng@...el.com>,
 "Annapurve, Vishal" <vannapurve@...gle.com>,
 "jroedel@...e.de" <jroedel@...e.de>, "Miao, Jun" <jun.miao@...el.com>,
 "pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 02/21] x86/virt/tdx: Enhance tdh_mem_page_aug() to
 support huge pages



On 5/16/25 12:05, Yan Zhao wrote:
> On Wed, May 14, 2025 at 02:52:49AM +0800, Edgecombe, Rick P wrote:
>> On Thu, 2025-04-24 at 11:04 +0800, Yan Zhao wrote:
>>> Enhance the SEAMCALL wrapper tdh_mem_page_aug() to support huge pages.
>>>
>>> Verify the validity of the level and ensure that the mapping range is fully
>>> contained within the page folio.
>>>
>>> As a conservative solution, perform CLFLUSH on all pages to be mapped into
>>> the TD before invoking the SEAMCALL TDH_MEM_PAGE_AUG. This ensures that any
>>> dirty cache lines do not write back later and clobber TD memory.
>>
>> This should have a brief background on why it doesn't use the arg - what is
>> deficient today. Also, an explanation of how it will be used (i.e. what types of
>> pages will be passed)
> Will do.
> 
>>>
>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>>> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
>>> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
>>> ---
>>>   arch/x86/virt/vmx/tdx/tdx.c | 11 ++++++++++-
>>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
>>> index f5e2a937c1e7..a66d501b5677 100644
>>> --- a/arch/x86/virt/vmx/tdx/tdx.c
>>> +++ b/arch/x86/virt/vmx/tdx/tdx.c
>>> @@ -1595,9 +1595,18 @@ u64 tdh_mem_page_aug(struct tdx_td *td, u64 gpa, int level, struct page *page, u
>>>   		.rdx = tdx_tdr_pa(td),
>>>   		.r8 = page_to_phys(page),
>>>   	};
>>> +	unsigned long nr_pages = 1 << (level * 9);
>>> +	struct folio *folio = page_folio(page);
>>> +	unsigned long idx = 0;
>>>   	u64 ret;
>>>   
>>> -	tdx_clflush_page(page);
>>> +	if (!(level >= TDX_PS_4K && level < TDX_PS_NR) ||
>>> +	    (folio_page_idx(folio, page) + nr_pages > folio_nr_pages(folio)))
>>> +		return -EINVAL;
>>
>> Shouldn't KVM not try to map a huge page in this situation? Doesn't seem like a
>> job for the SEAMCALL wrapper.
> Ok. If the decision is to trust KVM and all potential callers, it's reasonable
> to drop those checks.
> 
>>> +
>>> +	while (nr_pages--)
>>> +		tdx_clflush_page(nth_page(page, idx++));
>>
>> clflush_cache_range() is:
>> static void tdx_clflush_page(struct page *page)
>> {
>> 	clflush_cache_range(page_to_virt(page), PAGE_SIZE);
>> }
>>
>> So we have loops within loops...  Better to add an arg to tdx_clflush_page() or
>> add a variant that takes one.
> Ok.
> 
> One thing to note is that even with an extra arg, tdx_clflush_page() has to call
> clflush_cache_range() page by page because with
> "#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)",
> page virtual addresses are not necessarily contiguous.
> 
> What about Binbin's proposal [1]? i.e.,
> 
> while (nr_pages)
>       tdx_clflush_page(nth_page(page, --nr_pages));

What's the problem with using:

+       for (int i = 0; nr_pages; nr_pages--)
+               tdx_clflush_page(nth_page(page, i++))


The kernel now allows C99-style definition of variables inside a loop + 
it's clear how many times the loop has to be executed.
> 
> [1] https://lore.kernel.org/all/a7d0988d-037c-454f-bc6b-57e71b357488@linux.intel.com/
> 
>>> +
>>>   	ret = seamcall_ret(TDH_MEM_PAGE_AUG, &args);
>>>   
>>>   	*ext_err1 = args.rcx;
>>
>