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Re: [Qemu-devel] [PATCH 2/5] trace: [all] Add "guest_vmem" event

From: Lluís Vilanova
Subject: Re: [Qemu-devel] [PATCH 2/5] trace: [all] Add "guest_vmem" event
Date: Sun, 20 Mar 2016 19:09:20 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Peter Maydell writes:

> On 18 March 2016 at 18:50, Lluís Vilanova <address@hidden> wrote:
>> Peter Maydell writes:
>>> Trying to trace physaddrs is very tricky -- in the case of
>>> a TLB hit there is no guarantee you can still identify the
>>> physaddr of what you're accessing (the guest might have
>>> changed the page tables and not invalidated the TLB).
>> I was looking at how to modify the soft TLB code to generate that information
>> for the trace event, but it requires changes at every TCG target. But in any
>> case, that should always provide the same phys address used by QEMU, so the
>> event info is "correct" in that sense. Or did I miss something?

> Consider the sequence:
>  * guest makes access to vaddr V, currently mapped to physaddr P1
>    (which is host address H)
>  * we put the mapping V -> H into QEMU's TLB
>  * guest changes its page tables so V now maps to P2,
>    but doesn't do a TLB flush
>  * guest makes another access to vaddr V
>  * we hit in QEMU's TLB, so we know V and H; but we don't
>    know P1 (because we don't record that in the TLB) and we
>    can't even get it by walking the page table because
>    at this point V maps to P2, not P1. (And for sw-TLB
>    guest archs like MIPS you can't even do a V-to-P lookup
>    in QEMU non-intrusively.)

> (This is often defined to be unpredictable or similar in the guest
> architecture. But a buggy guest might do it, and tracing the
> wrong thing would be pretty confusing if you were trying to
> track down that bug.)

Oh! Yes, I seem to remember that code path now, I checked it a really long time
ago. I was assuming that whenever this event is enabled at compile time, I would
have to modify QEMU's TLB to store the guest physical address (then used by the
tracing event).


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