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

From: James Hogan
Subject: Re: [Qemu-devel] [PATCH] target-mips: Fix RDHWR exception host PC
Date: Thu, 28 Apr 2016 10:01:42 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

On Thu, Apr 28, 2016 at 09:53:04AM +0100, Leon Alrae wrote:
> On 27/04/16 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.

Note also this sentence isn't actually very clear. preventing partial
inlining sounds like it stops inlining altogether, whereas actually I
meant that it seems to ensure full inlining of check_hwrena(). Perhaps
you could change it to ", and in practice seems to ensure the full
inlining of check_hwrena() on x86_64." when you apply it.

> > 
> > 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(-)
> Whoops, thanks for the fix. I'll send the pullreq soon, hopefully it's
> not too late for 2.6.



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