On Tue, Jun 05, 2012 at 07:13:54PM +0200, Andrea Arcangeli wrote: > On Wed, May 30, 2012 at 04:01:51PM -0400, Konrad Rzeszutek Wilk wrote: > > The only time the _PAGE_PSE (_PAGE_PAT) is set is when > > _PAGE_PCD | _PAGE_PWT are set. It is this ugly transformation > > of doing: > > > > if (pat_enabled && _PAGE_PWT | _PAGE_PCD) > > pte = ~(_PAGE_PWT | _PAGE_PCD) | _PAGE_PAT; > > > > and then writting the pte with the 7th bit set instead of the > > 2nd and 3rd to mark it as WC. There is a corresponding reverse too > > (to read the pte - so the pte_val calls) - so if _PAGE_PAT is > > detected it will remove the _PAGE_PAT and return the PTE as > > if it had _PAGE_PWT | _PAGE_PCD. > > > > So that little bit of code will need some tweaking - as it does > > that even if _PAGE_PRESENT is not set. Meaning it would > > transform your _PAGE_PAT to _PAGE_PWT | _PAGE_PCD. Gah! > > It looks like this is disabled in current upstream? > 8eaffa67b43e99ae581622c5133e20b0f48bcef1 Yup. But it is a temporary bandaid that I hope to fix soon. > > > OK. I can whip up a patch to deal with the 'Gah!' case easily if needed. > > That would help! But again it looks disabled in Xen? > > About linux host (no xen) when I decided to use PSE I checked this part: > > /* Set PWT to Write-Combining. All other bits stay the same */ > /* > * PTE encoding used in Linux: > * PAT > * |PCD > * ||PWT > * ||| > * 000 WB _PAGE_CACHE_WB > * 001 WC _PAGE_CACHE_WC > * 010 UC- _PAGE_CACHE_UC_MINUS > * 011 UC _PAGE_CACHE_UC > * PAT bit unused > */ > > I need to go read the specs pdf and audit the code against the specs > to be sure but if my interpretation correct, PAT is never set on linux > host (novirt) the way the relevant msr are programmed. > > If I couldn't use the PSE (/PAT) it'd screw with 32bit because I need > to poke a bit between _PAGE_BIT_DIRTY and _PAGE_BIT_GLOBAL to avoid > losing space on the swap entry, and there's just one bit in that range > (PSE). > > _PAGE_UNUSED1 (besides it's used by Xen) wouldn't work unless I change > the swp entry format for 32bit x86 reducing the max amount of swap > (conditional to CONFIG_AUTONUMA so it wouldn't be the end of the > world, plus the amount of swap on 32bit NUMA may not be so important) Yeah, I concur. I think stick with _PAGE_PAT (/PSE) and I can cook up the appropiate patch for it on the Xen side. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>