linux-kernel - Re: [PATCH v6 4/5] PCI: Extend isolated function probing to s390

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Date:   Fri, 22 Jul 2022 16:13:29 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 4/5] PCI: Extend isolated function probing to s390

On Tue, Jun 28, 2022 at 04:30:59PM +0200, Niklas Schnelle wrote:
> Like the jailhouse hypervisor s390's PCI architecture allows passing
> isolated PCI functions to an OS instance. As of now this is was not
> utilized even with multi-function support as the s390 PCI code makes
> sure that only virtual PCI busses including a function with devfn 0 are
> presented to the PCI subsystem. A subsequent change will remove this
> restriction.
> 
> Allow probing such functions by replacing the existing check for
> jailhouse_paravirt() with a new hypervisor_isolated_pci_functions()
> helper.
> 
> Cc: Jan Kiszka <jan.kiszka@...mens.com>
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>  drivers/pci/probe.c        | 2 +-
>  include/linux/hypervisor.h | 8 ++++++++
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index a18e07e6a7df..156dd13594b8 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2667,7 +2667,7 @@ int pci_scan_slot(struct pci_bus *bus, int devfn)
>  			 * a hypervisor which passes through individual PCI
>  			 * functions.
>  			 */
> -			if (!jailhouse_paravirt())
> +			if (!hypervisor_isolated_pci_functions())
>  				break;
>  		}
>  		fn = next_fn(bus, dev, fn);
> diff --git a/include/linux/hypervisor.h b/include/linux/hypervisor.h
> index fc08b433c856..33b1c0482aac 100644
> --- a/include/linux/hypervisor.h
> +++ b/include/linux/hypervisor.h
> @@ -32,4 +32,12 @@ static inline bool jailhouse_paravirt(void)
>  
>  #endif /* !CONFIG_X86 */
>  
> +static inline bool hypervisor_isolated_pci_functions(void)
> +{
> +	if (IS_ENABLED(CONFIG_S390))
> +		return true;
> +	else
> +		return jailhouse_paravirt();

It looks kind of wasteful that jailhouse_paravirt() searches the DT
for "jailhouse,cell" several times when I think that's an unchanging
property.

Obviously you didn't add that in this series, and s390 avoids that
cost anyway.  But the jailhouse folks might consider optimizing it
somehow.

> +}
> +
>  #endif /* __LINUX_HYPEVISOR_H */
> -- 
> 2.32.0
>