[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [Patch 1/3 v2] x86/irq: local_irq_restore() should not blindly popf
>>> On 21.10.13 at 20:37, Keir Fraser <keir.xen@xxxxxxxxx> wrote: > On 21/10/2013 19:30, "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx> wrote: > >>>> #define read_segment_register(name) \ >>>> ({ u16 __sel; \ >>>> @@ -159,15 +160,19 @@ static always_inline unsigned long __cmpxchg( >>>> #define local_irq_restore(x) \ >>>> ({ \ >>>> BUILD_BUG_ON(sizeof(x) != sizeof(long)); \ >>>> - asm volatile ( "push" __OS " %0 ; popf" __OS \ >>>> - : : "g" (x) : "memory", "cc" ); \ >>>> + asm volatile ( \ >>>> + "pushf" __OS "\n\t" \ >>>> + "and" __OS " %0, (%%" __OP "sp)\n\t" \ >>>> + "orw %1, (%%" __OP "sp)\n\t" \ >>>> + "popf" __OS "\n\t" : : "g" ( ~X86_EFLAGS_IF ), \ >>> Would this be better as a constant constraint ("i")? >> >> I was wondering what the best practice for this would be. >> >> In most cases, I would imagine that an immediate would be used. >> However, as this is a define and therefore forcibly inlined everywhere >> it is used, it is just possible that the compiler could find a >> ~X86_EFLAGS_IF already in context, and optimise down to an "and r64,r/m64". > > Oh, g includes i, I forgot that. Well your choice is best then. Sorry, but no. "g" also includes "m", and - the other operand of both operations is a memory operand already, so this one can't also be a memory one, - on a non-debug build (without frame pointers) an eventual %rsp-relative memory location would be broken due to the shifted stack offsets resulting from the PUSHF. Hence both constraints can at best be "ri". Further I have a hard time seeing how the "orw" used above can even have built successfully: If a register gets picked (which ought to be the common case), opcode suffix and register name ought to collide. And "orw" is a bad choice here anyway, in that this is a 2-byte write following an 8-byte one. And finally - what's the point of using __OS in new assembly constructs? I was actually considering cleaning up all this hard to read cruft, since we no longer care about the 32-bit case. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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