Re: [Xen-devel] Is: axe read_tscp pvops call. Was: Re: [RFC] ACPI S3 and Xen (suprisingly small\!).

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: H. Peter Anvin [mailto:hpa@xxxxxxxxx]
> Sent: Wednesday, October 17, 2012 11:35 AM
> To: Konrad Rzeszutek Wilk
> Cc: linux-acpi@xxxxxxxxxxxxxxx; x86@xxxxxxxxxx; 
> xen-devel@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; lenb@xxxxxxxxxx
> Subject: Re: [Xen-devel] Is: axe read_tscp pvops call. Was: Re: [RFC] ACPI S3 
> and Xen (suprisingly
> small\!).
> 
> On 10/17/2012 09:54 AM, Konrad Rzeszutek Wilk wrote:
> >>
> >> Could you do an audit for other pvops calls that have no users?  If
> >> the *only* user is lguest, we should talk about it, too...
> >
> > I can do that - but I don't want to be hasty here. There is a bit of
> > danger here - for example the read_pmc (or read_tsc) is not in use right
> > now. But it might be when one starts looking at making perf be able to
> > analyze the hypervisor (hand-waving the implementation details). So while
> > removing read_pmc now sounds good, it might be needed in the future.
> >
> 
> We do not keep a pvop around just because it "might be needed in the
> future".  That's just crazy.
> 
>       -hpa

It's a bit more complicated than that.  The problem is that if
any patch is ever submitted to the kernel that uses the rdtscp
instruction *in kernel space* in some clever way, the resultant
kernel may not behave as expected (depending on how the instruction
is used) on a 32-bit[1] PV kernel running on Xen, up to and including
the possibility of data corruption.

I don't know how one would implement it, but it's like a
BUILD_BUG_ON is needed if any kernel developer uses rdtscp
(one that never gets invoked by vdso code), that prints:

"WARNING: Please do not use this instruction in the kernel
without notifying the Xen maintainer as there is a possibility
it may behave unpredictably in some Xen environments.
See Documentation/.../xen_pv_limitations for detail."

The other virtualization-unsafe instructions may have similar
problems.

Just FYI...

Dan

[1] I _think_ this is not a problem on 64-bit kernels but
am not certain.

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