RE: [PATCH 11/12] PCI: hv: Add hypercalls to read/write MMIO space

["Michael Kelley (LINUX)" <mikelley@microsoft.com>]

From: Bjorn Helgaas <helgaas@kernel.org> Sent: Thursday, October 20, 2022 12:04 PM
> 
> On Thu, Oct 20, 2022 at 10:57:14AM -0700, Michael Kelley wrote:
> > To support PCI pass-thru devices in Confidential VMs, Hyper-V
> > has added hypercalls to read and write MMIO space. Add the
> > appropriate definitions to hyperv-tlfs.h and implement
> > functions to make the hypercalls. These functions are used
> > in a subsequent patch.
> >
> > Co-developed-by: Dexuan Cui <decui@microsoft.com>
> > Signed-off-by: Dexuan Cui <decui@microsoft.com>
> > Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> > ---
> >  arch/x86/include/asm/hyperv-tlfs.h  |  3 ++
> >  drivers/pci/controller/pci-hyperv.c | 62
> +++++++++++++++++++++++++++++++++++++
> >  include/asm-generic/hyperv-tlfs.h   | 22 +++++++++++++
> >  3 files changed, 87 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/hyperv-tlfs.h b/arch/x86/include/asm/hyperv-
> tlfs.h
> > index 3089ec3..f769b9d 100644
> > --- a/arch/x86/include/asm/hyperv-tlfs.h
> > +++ b/arch/x86/include/asm/hyperv-tlfs.h
> > @@ -117,6 +117,9 @@
> >  /* Recommend using enlightened VMCS */
> >  #define HV_X64_ENLIGHTENED_VMCS_RECOMMENDED		BIT(14)
> >
> > +/* Use hypercalls for MMIO config space access */
> > +#define HV_X64_USE_MMIO_HYPERCALLS			BIT(21)
> > +
> >  /*
> >   * CPU management features identification.
> >   * These are HYPERV_CPUID_CPU_MANAGEMENT_FEATURES.EAX bits.
> > diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> > index e7c6f66..02ebf3e 100644
> > --- a/drivers/pci/controller/pci-hyperv.c
> > +++ b/drivers/pci/controller/pci-hyperv.c
> > @@ -1054,6 +1054,68 @@ static int wslot_to_devfn(u32 wslot)
> >  	return PCI_DEVFN(slot_no.bits.dev, slot_no.bits.func);
> >  }
> >
> > +static void hv_pci_read_mmio(phys_addr_t gpa, int size, u32 *val)
> > +{
> > +	struct hv_mmio_read_input *in;
> > +	struct hv_mmio_read_output *out;
> > +	u64 ret;
> > +
> > +	/*
> > +	 * Must be called with interrupts disabled so it is safe
> > +	 * to use the per-cpu input argument page.  Use it for
> > +	 * both input and output.
> > +	 */
> > +	in = *this_cpu_ptr(hyperv_pcpu_input_arg);
> > +	out = *this_cpu_ptr(hyperv_pcpu_input_arg) + sizeof(*in);
> > +	in->gpa = gpa;
> > +	in->size = size;
> > +
> > +	ret = hv_do_hypercall(HVCALL_MMIO_READ, in, out);
> > +	if (hv_result_success(ret)) {
> > +		switch (size) {
> > +		case 1:
> > +			*val = *(u8 *)(out->data);
> > +			break;
> > +		case 2:
> > +			*val = *(u16 *)(out->data);
> > +			break;
> > +		default:
> > +			*val = *(u32 *)(out->data);
> > +			break;
> > +		}
> > +	} else
> > +		pr_err("MMIO read hypercall failed with status %llx\n", ret);
> 
> Too bad there's not more information to give the user/administrator
> here.  Seeing "MMIO read hypercall failed with status -5" in the log
> doesn't give many clues about where to look or who to notify.  I don't
> know what's even feasible, but driver name, device, address (gpa),
> size would all be possibilities.
> 

Good point.  We can pass the device as an input argument to hv_pci_read_mmio()
and hv_pci_write_mmio(), use dev_err() instead of pr_err(), and provide the
gpa and size in the message.   This is one of those errors that should never
happen, but when it does, having the additional info will be helpful for
debugging.  I'll do this in v2.

Michael