Re: [Qemu-devel] spapr_pci: Advertise BAR reallocation capability

[David Gibson]

On Thu, Jun 06, 2019 at 02:09:19PM +1000, Alexey Kardashevskiy wrote:
> The pseries guests do not normally allocate PCI resouces and rely on
> the system firmware doing so. Furthermore at least at some point in
> the past the pseries guests won't even be allowed to change BARs, probably
> it is still the case for phyp. So since the initial commit we have [1]
> which prevents resource reallocation.
> 
> This is not a problem until we want specific BAR alignments, for example,
> PAGE_SIZE==64k to make sure we can still map MMIO BARs directly. For
> the boot time devices we handle this in SLOF [2] but since QEMU's RTAS
> does not allocate BARs, the guest does this instead and does not align
> BARs even if Linux is given pci=resource_alignment=16@pci:0:0 as
> PCI_PROBE_ONLY makes Linux ignore alignment requests.
> 
> ARM folks added a dial to control PCI_PROBE_ONLY via the device tree [3].
> This makes use of the dial to advertise to the guest that we can handle
> BAR reassignments.
> 
> We do not remove the flag from [1] as pseries guests are still supported
> under phyp so having that removed may cause problems.
> 
> [1] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/platforms/pseries/setup.c?h=v5.1#n773
> [2] 
> https://git.qemu.org/?p=SLOF.git;a=blob;f=board-qemu/slof/pci-phb.fs;h=06729bcf77a0d4e900c527adcd9befe2a269f65d;hb=HEAD#l338
> [3] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f81c11af
> Signed-off-by: Alexey Kardashevskiy <address@hidden>

Changing a guest visible property, that could have a big effect on how
the guest behaves, without a machine version change seems... unwise.

> ---
>  hw/ppc/spapr.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 6dd8aaac3340..84d16f9edaca 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -1209,6 +1209,9 @@ static void spapr_dt_chosen(SpaprMachineState *spapr, 
> void *fdt)
>          _FDT(fdt_setprop_string(fdt, chosen, "stdout-path", stdout_path));
>      }
>  
> +    /* We can deal with BAR reallocation just fine, advertise it to the 
> guest */
> +    _FDT(fdt_setprop_cell(fdt, chosen, "linux,pci-probe-only", 0));
> +
>      spapr_dt_ov5_platform_support(spapr, fdt, chosen);
>  
>      g_free(stdout_path);

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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