Re: [Qemu-devel] [RFC] pseries: Enable in-kernel H_LOGICAL_CI_{LOAD, STORE} implementations

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Feb 04, 2015 at 08:19:06AM +1100, Paul Mackerras wrote:
> On Tue, Feb 03, 2015 at 05:10:51PM +1100, David Gibson wrote:
> > qemu currently implements the hypercalls H_LOGICAL_CI_LOAD and
> > H_LOGICAL_CI_STORE as PAPR extensions.  These are used by the SLOF firmware
> > for IO, because performing cache inhibited MMIO accesses with the MMU off
> > (real mode) is very awkward on POWER.
> > 
> > This approach breaks when SLOF needs to access IO devices implemented
> > within KVM instead of in qemu.  The simplest example would be virtio-blk
> > using an iothread, because the iothread / dataplane mechanism relies on
> > an in-kernel implementation of the virtio queue notification MMIO.
> > 
> > To fix this, an in-kernel implementation of these hypercalls has been made,
> > however, the hypercalls still need to be enabled from qemu.  This performs
> > the necessary calls to do so.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> 
> [snip]
> 
> > +    ret1 = kvmppc_enable_hcall(kvm_state, H_LOGICAL_CI_LOAD);
> > +    if (ret1 != 0) {
> > +        fprintf(stderr, "Warning: error enabling H_LOGICAL_CI_LOAD in KVM:"
> > +                " %s\n", strerror(errno));
> > +    }
> > +
> > +    ret2 = kvmppc_enable_hcall(kvm_state, H_LOGICAL_CI_STORE);
> > +    if (ret2 != 0) {
> > +        fprintf(stderr, "Warning: error enabling H_LOGICAL_CI_STORE in KVM:"
> > +                " %s\n", strerror(errno));
> > +     }
> > +
> > +    if ((ret1 != 0) || (ret2 != 0)) {
> > +        fprintf(stderr, "Warning: Couldn't enable H_LOGICAL_CI_* in KVM, SLOF"
> > +                " may be unable to operate devices with in-kernel emulation\n");
> > +    }
> 
> You'll always get these warnings if you're running on an old (meaning
> current upstream) kernel, which could be annoying.

True.

> Is there any way
> to tell whether you have configured any devices which need the
> in-kernel MMIO emulation and only warn if you have?

In theory, I guess so.  In practice I can't see how you'd enumerate
all devices that might require kernel intervention without something
horribly invasive.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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