On Mon, Jul 14, 2014 at 7:46 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Mon, Jul 14, 2014 at 3:23 PM, H. Peter Anvin <h...@zytor.com> wrote: >> On 07/14/2014 02:35 PM, Andy Lutomirski wrote: >>> Presumably the problem is here: >>> >>> ENTRY(xen_iret) >>> pushq $0 >>> 1: jmp hypercall_iret >>> ENDPATCH(xen_iret) >>> >>> This seems rather unlikely to work on the espfix stack. >>> >>> Maybe espfix64 should be disabled when running on Xen and Xen should >>> implement its own espfix64 in the hypervisor. >> >> Perhaps the first question is: is espfix even necessary on Xen? How >> does the Xen PV IRET handle returning to a 16-bit stack segment? >> > > Test case here: > > https://gitorious.org/linux-test-utils/linux-clock-tests/source/dbfe196a0f6efedc119deb1cdbb0139dbdf609ee: > > It's sigreturn_32 and sigreturn_64. Summary: > > (sigreturn_64 always fails unless my SS patch is applied. results > below for sigreturn_64 assume the patch is applied. This is on KVM > (-cpu host) on Sandy Bridge.) > > On Xen with espfix, both OOPS intermittently. > > On espfix-less kernels (Xen and non-Xen), 16-bit CS w/ 16-bit SS > always fails. Native (32-bit or 64-bit, according to the binary) CS > with 16-bit SS fails for sigreturn_32, but passes for sigreturn_64. I > find this somewhat odd. Native ss always passes. > > So I think that Xen makes no difference here, aside from the bug. > > That being said, I don't know whether Linux can do espfix64 at all > when Xen is running -- for all I know, the IRET hypercall switches > stacks to a Xen stack.
Microcode is weird. Without espfix: [RUN] 64-bit CS (33), 32-bit SS (2b) SP: 8badf00d5aadc0de -> 8badf00d5aadc0de [OK] all registers okay [RUN] 32-bit CS (23), 32-bit SS (2b) SP: 8badf00d5aadc0de -> 5aadc0de [OK] all registers okay [RUN] 16-bit CS (7), 32-bit SS (2b) SP: 8badf00d5aadc0de -> 5aadc0de [OK] all registers okay [RUN] 64-bit CS (33), 16-bit SS (f) SP: 8badf00d5aadc0de -> 8badf00d5aadc0de [OK] all registers okay [RUN] 32-bit CS (23), 16-bit SS (f) SP: 8badf00d5aadc0de -> 5ae3c0de [FAIL] Reg 15 mismatch: requested 0x8badf00d5aadc0de; got 0x5ae3c0de [RUN] 16-bit CS (7), 16-bit SS (f) SP: 8badf00d5aadc0de -> 5ae3c0de [FAIL] Reg 15 mismatch: requested 0x8badf00d5aadc0de; got 0x5ae3c0de --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/