Messages in this thread | | | Date | Mon, 25 Aug 2014 14:22:35 +0200 | From | Juergen Gross <> | Subject | Re: [Xen-devel] [PATCH RFC 2/3] x86: Enable PAT to use cache mode translation tables |
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On 08/22/2014 11:32 AM, Jan Beulich wrote: >>>> On 19.08.14 at 15:25, <JGross@suse.com> wrote: >> @@ -118,8 +167,14 @@ void pat_init(void) >> PAT(4, WB) | PAT(5, WC) | PAT(6, UC_MINUS) | PAT(7, UC); >> >> /* Boot CPU check */ >> - if (!boot_pat_state) >> + if (!boot_pat_state) { >> rdmsrl(MSR_IA32_CR_PAT, boot_pat_state); >> + /* >> + * Init cache mode tables before writing MSR to give Xen a >> + * chance to correct the changes when doing the write. >> + */ > > This comment seems pretty odd to me: For one, a PV guest on Xen > shouldn't be trying to write PAT MSR at all under the current ABI > (the write will be ignored, yes, but accompanied with a warning > message, which PV kernels - by the mere fact that they're PV - > should try to avoid). And then "correct the changes" both gives > the impression as if they were wrong and as if some of what the > kernel writes may be under the kernel's control. Hence I think this > code and comment should either be consistently assuming that the > kernel has no control at all, or should read back the value after > having written it, and set the internal tables based on the value > read back.
I think the latter alternative is the better one. I'll change the patch.
Juergen
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