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Re: [Qemu-devel] [PATCH] target-mips: Fix RDHWR exception host PC

From: Aurelien Jarno
Subject: Re: [Qemu-devel] [PATCH] target-mips: Fix RDHWR exception host PC
Date: Thu, 28 Apr 2016 10:51:28 +0200
User-agent: Mutt/1.5.24 (2015-08-30)

On 2016-04-27 23:21, James Hogan wrote:
> Commit b00c72180c36 ("target-mips: add PC, XNP reg numbers to RDHWR")
> changed the rdhwr helpers to use check_hwrena() to check the register
> being accessed is enabled in CP0_HWREna when used from user mode. If
> that check fails an EXCP_RI exception is raised at the host PC
> calculated with GETPC().
> However check_hwrena() may not be fully inlined as the
> do_raise_exception() part of it is common regardless of the arguments.
> This causes GETPC() to calculate the address in the call in the helper
> instead of the generated code calling the helper. No TB will be found
> and the EPC reported with the resulting guest RI exception points to the
> beginning of the TB instead of the RDHWR instruction.
> We can't reliably force check_hwrena() to be inlined, and converting it
> to a macro would be ugly, so instead pass the host PC in as an argument,
> with each rdhwr helper passing GETPC(). This should avoid any dependence
> on compiler behaviour, and in practice seems to prevent the partial
> inlining of check_hwrena() on x86_64.
> This issue causes failures when running a MIPS KVM (trap & emulate)
> guest in a MIPS QEMU TCG guest, as the inner guest kernel will do a
> RDHWR of counter, which is disabled in the outer guest's CP0_HWREna by
> KVM so it can emulate the inner guest's counter. The emulation fails and
> the RI exception is passed to the inner guest.
> Fixes: b00c72180c36 ("target-mips: add PC, XNP reg numbers to RDHWR")
> Signed-off-by: James Hogan <address@hidden>
> Cc: Leon Alrae <address@hidden>
> Cc: Yongbok Kim <address@hidden>
> Cc: Aurelien Jarno <address@hidden>
> ---
>  target-mips/op_helper.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)

Thanks for the detailed analysis. The other solution would have been to
declare the function as __attribute__((__always_inline__)), but I think
your solution is even better.

Reviewed-by: Aurelien Jarno <address@hidden>

Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net

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