Re: [RFC PATCH 02/21] x86/virt/tdx: Enhance tdh_mem_page_aug() to support huge pages

[Nikolay Borisov]

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@xxxxxxxxx>
> > > Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > > ---
> > >   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@xxxxxxxxxxxxxxx/
>
> > > +
> > >   	ret = seamcall_ret(TDH_MEM_PAGE_AUG, &args);
> > >   
> > >   	*ext_err1 = args.rcx;