Re: [PATCH 00/18] CXL RAM and the 'Soft Reserved' => 'System RAM' default

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On Tue, Feb 14, 2023 at 04:51:53PM -0500, Gregory Price wrote:
> On Tue, Feb 14, 2023 at 09:18:24PM +0000, Jonathan Cameron wrote:
> > On Tue, 14 Feb 2023 14:01:23 -0500
> > Gregory Price <gregory.price@xxxxxxxxxxxx> wrote:
> > 
> > Could you test it with TCG (just drop --enable-kvm)?  We have a known
> > limitation with x86 instructions running out of CXL emulated memory
> > (side effect of emulating the interleave).  You'll need a fix even on TCG
> > for the corner case of an instruction bridging from normal ram to cxl memory.
> > https://lore.kernel.org/qemu-devel/20230206193809.1153124-1-richard.henderson@xxxxxxxxxx/
> > 
> > Performance will be bad, but so far this is only way we can do it correctly.
> > 
> > Jonathan
> > 
> 
> Siiiggghh... i had this patch and dropped --enable-kvm, but forgot to
> drop "accel=kvm" from the -machine line
> 
> This was the issue.
> 
> And let me tell you, if you numactl --membind=1 python, it is
> IMPRESSIVELY slow.  I wonder if it's even hitting a few 100k
> instructions a second.
> 
> 
> This appears to be the issue.  When I get a bit more time, try to dive
> into the deep dark depths of qemu memory regions to see how difficult
> a non-mmio fork might be, unless someone else is already looking at it.
> 
> ~Gregory

Just clarifying one thing:  Even with the patch, KVM blows up.
Disabling KVM fixes this entirely.  I haven't tested without KVM but
with the patch, i will do that now.



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