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Date: Wed, 13 Jun 2007 16:19:04 -0600 From: ebiederm@...ssion.com (Eric W. Biederman) To: Jesse Barnes <jbarnes@...tuousgeek.org> Cc: Jesse Barnes <jesse.barnes@...el.com>, Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org, Justin Piszcz <jpiszcz@...idpixels.com> Subject: Re: [PATCH] trim memory not covered by WB MTRRs Jesse Barnes <jbarnes@...tuousgeek.org> writes: > On Tuesday, June 12, 2007 6:11:21 Eric W. Biederman wrote: >> Jesse Barnes <jesse.barnes@...el.com> writes: >> > On some machines, buggy BIOSes don't properly setup WB MTRRs to >> > cover all available RAM, meaning the last few megs (or even gigs) >> > of memory will be marked uncached. Since Linux tends to allocate >> > from high memory addresses first, this causes the machine to be >> > unusably slow as soon as the kernel starts really using memory >> > (i.e. right around init time). >> > >> > This patch works around the problem by scanning the MTRRs at >> > boot and figuring out whether the current end_pfn value (setup >> > by early e820 code) goes beyond the highest WB MTRR range, and >> > if so, trimming it to match. A fairly obnoxious KERN_WARNING >> > is printed too, letting the user know that not all of their >> > memory is available due to a likely BIOS bug. >> >> A quick update. This patch is horribly incorrect on a socket F >> opteron/Athlon 64 with memory above 4GB. >> >> In particular those cpus are capable of mapping all of memory >> above 4GB as write back without using a single MTRR. >> >> So examining MTRRs is insufficient. > > Hm, yuck. What do you suggest? Should we only run this check when Intel > chips are present? Checking only the bottom 4G isn't sufficient since we've > seen platforms that have issues above that range... My gut feel says that we need to call a function that is potentially cpu specific, older AMD cpus and Intel cpus can just use the generic mtrr code. I would also suggest we build a list of ranges of write-back memory. Which until we handle overlapping MTRRs in the generic MTRR case is just the write-back MTRRs. Then we get the data in a linux specific form we can check the linux specific data structure against the e820 map. I don't think that is going to much harder and it allows for creative cpu designers. Although this suggests that we want to worry about all memory holes as well. Because I have seen at least one system which failed to cover the lower 4G with MTRRs. While everything above 4G was fine. Eric - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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