Re: [PATCH] x86/mm/cpa: set PAGE_KERNEL in __set_pages_p()

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Tue, May 10, 2022 at 12:47:11AM +0000, Edgecombe, Rick P wrote:
> On Fri, 2022-05-06 at 14:19 +0900, Hyeonggon Yoo wrote:
> > __set_pages_np() not only clears _PAGE_PRESENT and _PAGE_RW, but also
> > clears _PAGE_GLOBAL to avoid confusing _PAGE_GLOBAL as _PAGE_PROTNONE
> > when the PTE is not present.
> > 
> > Common usage for __set_pages_p() is to call it after
> > __set_pages_np().
> > Therefore calling __set_pages_p() after __set_pages_np() clears
> > _PAGE_GLOBAL, making it unable to globally shared in TLB.
> > 
> > As they are called by set_direct_map_{invalid,default}_noflush(),
> > pages in direct map cannot be globally shared in TLB after being used
> > by
> > vmalloc, secretmem, and hibernation.
> > 
> > So set PAGE_KERNEL isntead of __pgprot(_PAGE_PRESENT | _PAGE_RW) in
> > __set_pages_p().
> 
> Nice find. I think we can't always set PAGE_KERNEL also because of the
> PTI code. It sometimes wants the direct map to be non global.
>

Thanks for review!

IIUC __pgprot_mask() already clears _PAGE_GLOBAL from PAGE_KERNEL
when PTI is used.

#define __pgprot_mask(x)        __pgprot((x) & __default_kernel_pte_mask)
#define PAGE_KERNEL             __pgprot_mask(__PAGE_KERNEL | _ENC)

But yeah, it seems PAGE_KERNEL is too much for this.

> Maybe something like this?
> (_PAGE_PRESENT | _PAGE_RW | _PAGE_GLOBAL) & __default_kernel_pte_mask
>

What about __pgprot_mask(_PAGE_PRESENT | _PAGE_RW | _PAGE_GLOBAL)?

> That would add back in a little of the "default global" behavior that
> was removed in d1440b2, but I think it should be ok because it is
> limited to the direct map. 

> Otherwise, I wonder if the existing global
> clearing logic is really needed.

I think it's still needed. pte_present() returning true due to _PAGE_PROTNONE
after __set_pages_np() simply doesn't make sense. No?

-- 
Thanks,
Hyeonggon