Messages in this thread | | | Date | Wed, 06 Jun 2018 15:53:02 -0700 | Subject | Re: [PATCH v2 7/8] x86/segments/32: Introduce CPU_NUMBER segment | From | hpa@zytor ... |
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On June 6, 2018 12:07:15 PM PDT, Brian Gerst <brgerst@gmail.com> wrote: >On Wed, Jun 6, 2018 at 1:16 PM, Andy Lutomirski <luto@kernel.org> >wrote: >> On Wed, Jun 6, 2018 at 9:23 AM Chang S. Bae ><chang.seok.bae@intel.com> wrote: >>> >>> The new entry will be equivalent to that of x86-64 which >>> stores CPU number. The entry is placed in segment 23 in GDT >>> by bumping down 23-28 by one, which are all kernel-internal >>> segments and so have no impact on user space. >>> >>> CPU_NUMBER segment will always be at '%ss (USER_DS) + 80' >>> for the default (flat, initial) user space %ss. >> >> No, it won't :( This is because, on Xen PV, user code very >frequently >> sees a different, Xen-supplied "flat" SS value. This is definitely >> true right now on 64-bit, and I'm reasonably confident it's also the >> case on 32-bit. >> >> As it stands, as far as I can tell, we don't have a "cpu number" >> segment on 32-bit kernels. I see no compelling reason to add one, >and >> we should definitely not add one as part of the FSGSBASE series. I >> think the right solution is to rename the 64-bit segment to >> "CPU_NUMBER" and then have the rearrangement of the initialization >> code as a followup patch. The goal is to make the patches >> individually reviewable. As it stands, this patch adds some #defines >> without making them work, which is extra confusing. >> >> Given how many times we screwed it up, I really want the patch that >> moves the initialization of the 64-bit CPU number to be obviously >> correct and to avoid changing the sematics of anything except the >> actual CPU number fields during boot. >> >> So NAK to this patch, at least as part of the FSGSBASE series. >> >> (My apologies -- a bunch of this is because I along with everyone >else >> misunderstood the existing code.) > >The sole purpose of this segment is for the getcpu() function in the >VDSO. No other userspace code can rely on its presence or location. > >-- >Brian Gerst
Unfortunately that is not true in reality :( -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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