Re: [PATCH 4/4] x86: use pte_none() to test for empty PTE

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 07/14/2016 06:47 AM, Vlastimil Babka wrote:
> So, this might be just because I know next to nothing about (para)virt,
> but...
> 
> in arch/x86/include/asm/paravirt.h, pte_val is implemented via some
> pvops, which suggests that obtaining a pte value is different than just
> reading it from memory. But I don't see pte_none() defined to be using
> this on paravirt, and it shares (before patch 2/4) the "return !pte.pte"
> implementation, AFAICS?
> 
> So that itself is suspicious to me. And now that this patches does
> things like this:
> 
> -              if (pte_val(*pte)) {
> +              if (!pte_none(*pte)) {
> 
> So previously on paravirt these tests would read pte via the pvops, and
> now they won't. Is that OK?

I've cc'd a few Xen guys.  I think they're the only ones that would care.

But, as far as I can tell, the Xen pte_val() will take a _PAGE_PRESENT
PTE and muck with it.  But its answer will never differ for an all 0 PTE
from !pte_none() because that PTE does not have _PAGE_PRESENT set.

It does seem fragile that Xen is doing it this way, but I guess it works.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>



[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux]     [Linux OMAP]     [Linux MIPS]     [ECOS]     [Asterisk Internet PBX]     [Linux API]