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

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Message-ID: <aCb/zC9dphPOuHgB@yzhao56-desk.sh.intel.com>
Date: Fri, 16 May 2025 17:05:16 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
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Subject: Re: [RFC PATCH 02/21] x86/virt/tdx: Enhance tdh_mem_page_aug() to
 support huge pages

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));

[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;
>