linux-kernel - Re: [PATCH 23/32] KVM: s390: pci: handle refresh of PCI translations

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Date:   Thu, 16 Dec 2021 15:39:37 +0100
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>, linux-s390@...r.kernel.org
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        farman@...ux.ibm.com, borntraeger@...ux.ibm.com, hca@...ux.ibm.com,
        gor@...ux.ibm.com, gerald.schaefer@...ux.ibm.com,
        agordeev@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        imbrenda@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com, thuth@...hat.com,
        pasic@...ux.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 23/32] KVM: s390: pci: handle refresh of PCI translations

On Tue, 2021-12-14 at 12:54 -0500, Matthew Rosato wrote:
> On 12/14/21 11:59 AM, Pierre Morel wrote:
> > 
> > On 12/7/21 21:57, Matthew Rosato wrote:
> > > Add a routine that will perform a shadow operation between a guest
> > > and host IOAT.  A subsequent patch will invoke this in response to
> > > an 04 RPCIT instruction intercept.
> > > 
> > > Signed-off-by: Matthew Rosato <mjrosato@...ux.ibm.com>
> > > ---
> > >   arch/s390/include/asm/kvm_pci.h |   1 +
> > >   arch/s390/include/asm/pci_dma.h |   1 +
> > >   arch/s390/kvm/pci.c             | 191 ++++++++++++++++++++++++++++++++
> > >   arch/s390/kvm/pci.h             |   4 +-
> > >   4 files changed, 196 insertions(+), 1 deletion(-)
> > > 
---8<---
> > 
> > > +
> > > +int kvm_s390_pci_refresh_trans(struct kvm_vcpu *vcpu, unsigned long req,
> > > +                   unsigned long start, unsigned long size)
> > > +{
> > > +    struct zpci_dev *zdev;
> > > +    u32 fh;
> > > +    int rc;
> > > +
> > > +    /* If the device has a SHM bit on, let userspace take care of 
> > > this */
> > > +    fh = req >> 32;
> > > +    if ((fh & aift.mdd) != 0)
> > > +        return -EOPNOTSUPP;
> > 
> > I think you should make this check in the caller.
> 
> OK
> 
> > > +
> > > +    /* Make sure this is a valid device associated with this guest */
> > > +    zdev = get_zdev_by_fh(fh);
> > > +    if (!zdev || !zdev->kzdev || zdev->kzdev->kvm != vcpu->kvm)
> > > +        return -EINVAL;
> > > +
> > > +    /* Only proceed if the device is using the assist */
> > > +    if (zdev->kzdev->ioat.head[0] == 0)
> > > +        return -EOPNOTSUPP;
> > 
> > Using the assist means using interpretation over using interception and 
> > legacy vfio-pci. right?
> 
> Right - more specifically that the IOAT assist feature was never set via 
> the vfio feature ioctl, so we can't handle the RPCIT for this device and 
> so throw to userspace.
> 
> The way the QEMU series is being implemented, a device using 
> interpretation will always have the IOAT feature set on.
> 
> > > +
> > > +    rc = dma_table_shadow(vcpu, zdev, start, size);
> > > +    if (rc > 0)
> > > +        rc = zpci_refresh_trans((u64) zdev->fh << 32, start, size);
> > 
> > Here you lose the status reported by the hardware.
> > You should directly use __rpcit(fn, addr, range, &status);
> 
> OK, I can have a look at doing this.
> 
> @Niklas thoughts on how you would want this exported.  Renamed to 
> zpci_rpcit or so?

Hmm with using __rpcit() directly we would lose the error reporting in
s390dbf and this ist still kind of a RPCIT in the host. How about we
add the status as an out parameter to zpci_refresh_trans()? But yes if
you prefer to use __rpcit() directly I would rename it to zpci_rpcit().

> 

---8<---