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

[Bjorn Helgaas]

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@xxxxxxxxxxxxx>
> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Signed-off-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> ---
>  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.

Bjorn