On Tue, Jun 9, 2015 at 2:33 AM, Ingo Molnar <[email protected]> wrote: > > * Andy Lutomirski <[email protected]> wrote: > >> On Mon, Jun 8, 2015 at 1:34 AM, Ingo Molnar <[email protected]> wrote: >> > So the SYSENTER instruction is pretty quirky and it has different behavior >> > depending on bitness and CPU maker. >> > >> > Yet we create a false sense of coherency by naming it >> > 'ia32_sysenter_target' >> > in both of the cases. >> > >> > Split the name into its two uses: >> > >> > ia32_sysenter_target (32) -> entry_SYSENTER_32 >> > ia32_sysenter_target (64) -> entry_SYSENTER_compat >> > >> >> Now that I'm rebasing my pile on top of this, I have a minor gripe >> about this one. There are (in my mind, anyway), two SYSENTER >> instructions: the 32-bit one and the 64-bit one. (That is, there's >> SYSENTER32, which happens when you do SYSENTER in 32-bit or compat >> mode, and SYSENTER64, which happens when you do SYSENTER in long >> mode.) SYSENTER32, from user code's perspective, does the same thing >> in either case [1]. That means that it really does make sense that >> we'd have two implementations of the same entry point, one written in >> 32-bit asm and one written in 64-bit asm. >> >> The patch I'm rebasing merges the two wrmsrs to MSR_IA32_SYSENTER, and >> this change makes it uglier. >> >> [1] Sort of. We probably have differently nonsensical calling >> conventions, but that's our fault and has nothing to do with the >> hardware. > > Did you intend to merge these two wrmsr()s: > > #ifdef CONFIG_X86_64 > void syscall_init(void) > { > ... > wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)entry_SYSENTER_compat); > ... > } > #endif > > #ifdef CONFIG_X86_32 > void enable_sep_cpu(void) > { > ... > wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long)entry_SYSENTER_32, 0); > ... > } > > ... and the new bifurcated names preserve the #ifdef, right?
Exactly. > > So I mostly agree with you, but still I'm a bit torn about this, for the > following > reason: > > - SYSENTER on a 32-bit kernel behaves a bit differently from SYSENTER on a > 64-bit > kernel: for example on 32-bit kernels we'll return with SYSEXIT, while on > 64-bit kernels we return with SYSRET. The difference is small but > user-space > observable: for example EDX is 0 on SYSRET while it points to > ->sysenter_return > in the SYSEXIT case. > > This kind of user-observable assymmetry does not exist for other unified > syscall > ABIs, such as the INT80 method. > > So I think that despite having to preserve a small non-unified #ifdef for this > initialization, we are still better off naming the two entry points > differently, > along the pattern we use, because the behavior is slightly different > depending on > the bitness of the kernel. > Fair enough. This is certainly not a big deal either way. Maybe when this really gets cleaned up, we can merge the entry points again. --Andy > Thanks, > > Ingo -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
