Messages in this thread |  | | Subject | Re: [PATCH] x86/entry/64/compat: Preserve r8-r11 in int $0x80 | From | Denys Vlasenko <> | Date | Wed, 18 Apr 2018 19:13:04 +0200 |
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On 04/18/2018 06:53 PM, Andy Lutomirski wrote: > On Tue, Apr 17, 2018 at 8:00 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote: >> This means that the new behavior is there for some 8 years already. >> Whoever was impacted by it, probably already switched to the new ABI. >> >> Current ABI is "weaker", it allows kernel to save fewer registers. >> >> Which is generally a good thing, since saving/restoring things cost >> cycles, and sometimes painful on entry paths where you may desperately >> need a scratch register or two. (Recall this one? - >> ... >> movq %rsp, PER_CPU_VAR(rsp_scratch) >> movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp >> /* Construct struct pt_regs on stack */ >> pushq $__USER_DS /* pt_regs->ss */ >> pushq PER_CPU_VAR(rsp_scratch) /* pt_regs->sp */ >> ... >> wouldn't it be _great_ if one of GPRs would be available here >> to hold userspace %rsp? >> ) > > But this is the int $0x80 entry, which uses the stack sanely and > doesn't have this problem at all.
It was a general point why not committing to save every register may help on the callee (in this case kernel) side.
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