Re: [Qemu-devel] [PATCH 1/2] pcie: Add a simple PCIe ACS (Access Control Services) helper function

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 23 Jan 2019 20:46:14 +0100
Knut Omang <knut.omang@oracle.com> wrote:

> On Wed, 2019-01-23 at 12:04 -0700, Alex Williamson wrote:
> > On Wed, 23 Jan 2019 19:27:59 +0100
> > Knut Omang <knut.omang@oracle.com> wrote:
> >   
> > > Add a helper function to add PCIe capability for Access Control Services (ACS)
> > > ACS support in the associated root port is a prerequisite to be able to do useful
> > > passthrough with VFIO without Alex Williamson's pcie_acs_override kernel patch.  
> > 
> > Define "useful".  We can certainly still assign single function PFs to
> > an L2 guest, or multi-function so long as all the functions are
> > assigned.  I won't deny that it's problematic, but it's a virtual
> > topology that can be adjusted, so I think this is overstating things a
> > bit.
> >   
> > > Signed-off-by: Knut Omang <knut.omang@oracle.com>
> > > ---
> > >  hw/pci/pcie.c              | 14 ++++++++++++++
> > >  include/hw/pci/pcie.h      |  1 +
> > >  include/hw/pci/pcie_regs.h |  4 ++++
> > >  3 files changed, 19 insertions(+)
> > > 
> > > diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c
> > > index 230478f..18feff5 100644
> > > --- a/hw/pci/pcie.c
> > > +++ b/hw/pci/pcie.c
> > > @@ -906,3 +906,17 @@ void pcie_ats_init(PCIDevice *dev, uint16_t offset)
> > >  
> > >      pci_set_word(dev->wmask + dev->exp.ats_cap + PCI_ATS_CTRL, 0x800f);
> > >  }
> > > +
> > > +/* Add an ACS (Access Control Services) capability */
> > > +void pcie_acs_init(PCIDevice *dev, uint16_t offset, uint8_t egress_ctrl_vec_sz)
> > > +{
> > > +    int ectrl_words = (egress_ctrl_vec_sz + 31) & ~31;
> > > +    pcie_add_capability(dev, PCI_EXT_CAP_ID_ACS, PCI_ACS_VER,
> > > +                        offset, PCI_ACS_SIZEOF + ectrl_words);  
> > 
> > The egress control vector is only valid if the egress control
> > capability is enabled, which is not set below, so this just seems to
> > waste config space and introduces a meaningless function arg.  
> 
> I think my intention way back when I implemented this was to provide it as a skeleton
> for further detailing later, I use it with ectrl_words = 0 in the root port.
> 
> > > +    pci_set_word(dev->config + offset + PCI_ACS_CAP,
> > > +                 PCI_ACS_SV | PCI_ACS_TB | PCI_ACS_RR | PCI_ACS_CR | PCI_ACS_UF);  
> > 
> > Some of these bits are only valid for downstream ports, it would
> > violate the spec to set them on and endpoint.  
> 
> I must admit I haven't really dived deep here - I just set the cap bits that one of my
> servers sets for it's root ports - this is the one I used as model:
> 
> 00:02.0 PCI bridge: Intel Corporation 5520/5500/X58 I/O Hub PCI Express Root Port 1 (rev
> 22) (prog-if 00 [Normal decode])
> 00:02.0 0604: 8086:3409 (rev 22)
> 
> > > +    pci_set_word(dev->config + offset + PCI_ACS_CTRL,
> > > +                 PCI_ACS_SV | PCI_ACS_TB | PCI_ACS_RR | PCI_ACS_CR | PCI_ACS_UF);  
> > 
> > The default values of the control register bits is zero, so we
> > shouldn't be setting it here and we should have a reset hook to clear
> > it.  
> 
> I agree - I'll implement it,
> thanks!
> 
> Knut
> 
> >   
> > > +    /* Make CTRL register writable */
> > > +    memset(dev->wmask + offset + PCI_ACS_CTRL, 0xff, 2);

While you're at it, it doesn't make sense to set unimplemented control
bits as writable, this should match the bits set in the capabilities
register.  Thanks,

Alex

> > > +}
> > > diff --git a/include/hw/pci/pcie.h b/include/hw/pci/pcie.h
> > > index 5b82a0d..c2da148 100644
> > > --- a/include/hw/pci/pcie.h
> > > +++ b/include/hw/pci/pcie.h
> > > @@ -129,6 +129,7 @@ void pcie_add_capability(PCIDevice *dev,
> > >  void pcie_sync_bridge_lnk(PCIDevice *dev);
> > >  
> > >  void pcie_ari_init(PCIDevice *dev, uint16_t offset, uint16_t nextfn);
> > > +void pcie_acs_init(PCIDevice *dev, uint16_t offset, uint8_t egress_ctrl_vec_sz);
> > >  void pcie_dev_ser_num_init(PCIDevice *dev, uint16_t offset, uint64_t ser_num);
> > >  void pcie_ats_init(PCIDevice *dev, uint16_t offset);
> > >  
> > > diff --git a/include/hw/pci/pcie_regs.h b/include/hw/pci/pcie_regs.h
> > > index ad4e780..5e7409c 100644
> > > --- a/include/hw/pci/pcie_regs.h
> > > +++ b/include/hw/pci/pcie_regs.h
> > > @@ -175,4 +175,8 @@ typedef enum PCIExpLinkWidth {
> > >                                           PCI_ERR_COR_INTERNAL |         \
> > >                                           PCI_ERR_COR_HL_OVERFLOW)
> > >  
> > > +/* ACS */
> > > +#define PCI_ACS_VER			0x2
> > > +#define PCI_ACS_SIZEOF                  8
> > > +
> > >  #endif /* QEMU_PCIE_REGS_H */  
> >   
> 
>