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Re: User applications

To: "Maciej W. Rozycki" <>
Subject: Re: User applications
From: "Kevin D. Kissell" <>
Date: Mon, 8 Jan 2001 16:21:18 +0100
Cc: <>, "Carsten Langgaard" <>, "Michael Shmulevich" <>
References: <>
----- Original Message -----
From: "Maciej W. Rozycki" <>
To: "Kevin D. Kissell" <>
Cc: <>; "Carsten Langgaard" <>;
"Michael Shmulevich" <>
Sent: Monday, January 08, 2001 4:07 PM
Subject: Re: User applications

> On Mon, 8 Jan 2001, Kevin D. Kissell wrote:
> > Back in the Ancient Old Days of System V, every architecture
> > had an architecture-specific system call entry, the first parameter
> > of which expressed what needed to be done.  Do we have
> > such a thing in Linux?   That would be the logical place to
> > things like cache flush and the atomic operations that were
> > being discussed here a couple of weeks ago.
>  The only case caches need to be synchronized is modifying some code.

Which just happens to be the case under discussion here.

>The  ptrace syscall does it automatically for text writes -- it's needed
> used by gdb to set breakpoints, for example.  For other code there is
> cacheflush() which allows you to flush a cache range relevant to a given
> virtual address (I see it's not implemented very well at the moment).
>  Obviously, you don't want to allow unprivileged users to flush caches as
> a whole as it could lead to a DoS.

By that logic, we should not allow users to allocate more virtual
memory than there is physical memory in the system!  A pathological
swap program is arguably far a far worse denial of service attack
than flushing the caches - so long as by "flush" we mean invalidate
with writeback (on copyback caches), of course.

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