Re: [PATCH 4/5] hw/i386/acpi-build: Deny control on PCIe Native Hot-plug in _OSC

[Julia Suvorova]

On Wed, Nov 10, 2021 at 2:58 PM Igor Mammedov <imammedo@redhat.com> wrote:
>
> On Wed, 10 Nov 2021 02:21:34 -0500
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> > On Wed, Nov 10, 2021 at 06:30:13AM +0100, Julia Suvorova wrote:
> > > There are two ways to enable ACPI PCI Hot-plug:
> > >
> > >         * Disable the Hot-plug Capable bit on PCIe slots.
> > >
> > > This was the first approach which led to regression [1-2], as
> > > I/O space for a port is allocated only when it is hot-pluggable,
> > > which is determined by HPC bit.
> > >
> > >         * Leave the HPC bit on and disable PCIe Native Hot-plug in _OSC
> > >           method.
> > >
> > > This removes the (future) ability of hot-plugging switches with PCIe
> > > Native hotplug since ACPI PCI Hot-plug only works with cold-plugged
> > > bridges. If the user wants to explicitely use this feature, they can
> > > disable ACPI PCI Hot-plug with:
> > >         --global ICH9-LPC.acpi-pci-hotplug-with-bridge-support=off
> > >
> > > Change the bit in _OSC method so that the OS selects ACPI PCI Hot-plug
> > > instead of PCIe Native.
> > >
> > > [1] https://gitlab.com/qemu-project/qemu/-/issues/641
> > > [2] https://bugzilla.redhat.com/show_bug.cgi?id=2006409
> > >
> > > Signed-off-by: Julia Suvorova <jusual@redhat.com>
> > > ---
> > >  hw/i386/acpi-build.c | 12 ++++++++----
> > >  1 file changed, 8 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> > > index a3ad6abd33..a2f92ab32b 100644
> > > --- a/hw/i386/acpi-build.c
> > > +++ b/hw/i386/acpi-build.c
> > > @@ -1337,7 +1337,7 @@ static void build_x86_acpi_pci_hotplug(Aml *table, 
> > > uint64_t pcihp_addr)
> > >      aml_append(table, scope);
> > >  }
> > >
> > > -static Aml *build_q35_osc_method(void)
> > > +static Aml *build_q35_osc_method(bool acpi_pcihp)
> > >  {
> > >      Aml *if_ctx;
> > >      Aml *if_ctx2;
> > > @@ -1345,6 +1345,7 @@ static Aml *build_q35_osc_method(void)
> > >      Aml *method;
> > >      Aml *a_cwd1 = aml_name("CDW1");
> > >      Aml *a_ctrl = aml_local(0);
> > > +    const uint64_t hotplug = acpi_pcihp ? 0x1E : 0x1F;
> >
> > drop this variable and open-code at use point below.
> > As it is the comment is misplaced.
> > Also a slightly better way to write this:
> > 0x1E | (acpi_pcihp ? 0x0 : 0x1)
> >
> >
> >
> > >
> > >      method = aml_method("_OSC", 4, AML_NOTSERIALIZED);
> > >      aml_append(method, aml_create_dword_field(aml_arg(3), aml_int(0), 
> > > "CDW1"));
> >
> > So what acpi_pcihp does is enable/disable native pcie hotplug.
> > How about we call the option exactly that?
> >
> >
> > > @@ -1359,8 +1360,9 @@ static Aml *build_q35_osc_method(void)
> > >      /*
> > >       * Always allow native PME, AER (no dependencies)
> > >       * Allow SHPC (PCI bridges can have SHPC controller)
> > > +     * Disable PCIe Native Hot-plug if ACPI PCI Hot-plug is enabled.
> > >       */
> > > -    aml_append(if_ctx, aml_and(a_ctrl, aml_int(0x1F), a_ctrl));
> > > +    aml_append(if_ctx, aml_and(a_ctrl, aml_int(hotplug), a_ctrl));
> > >
> >
> > using the variable hotplug just made things confusing,
> > open-coding will be clearer.
> >
> >
> > >      if_ctx2 = aml_if(aml_lnot(aml_equal(aml_arg(1), aml_int(1))));
> > >      /* Unknown revision */
> > > @@ -1449,7 +1451,7 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
> > >          aml_append(dev, aml_name_decl("_CID", aml_eisaid("PNP0A03")));
> > >          aml_append(dev, aml_name_decl("_ADR", aml_int(0)));
> > >          aml_append(dev, aml_name_decl("_UID", 
> > > aml_int(pcmc->pci_root_uid)));
> > > -        aml_append(dev, build_q35_osc_method());
> > > +        aml_append(dev, build_q35_osc_method(pm->pcihp_bridge_en));
> > >          aml_append(sb_scope, dev);
> > >          if (mcfg_valid) {
> > >              aml_append(sb_scope, build_q35_dram_controller(&mcfg));
> >
> > One of the complaints of libvirt people was that the
> > name is confusing. It seems to say whether to describe bridges
> > in ACPI but it also disables native hotplug, but only for PCIe.
> >
> > Maybe we should address this with flags saying whether to enable each of
> > acpi,pcie,shpc hotplug separately...
>
> yep, mask with field defines would be better,
> but I'd hold off such change to post 6.2 time.
>
> >
> > > @@ -1565,7 +1567,9 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
> > >              if (pci_bus_is_express(bus)) {
> > >                  aml_append(dev, aml_name_decl("_HID", 
> > > aml_eisaid("PNP0A08")));
> > >                  aml_append(dev, aml_name_decl("_CID", 
> > > aml_eisaid("PNP0A03")));
> > > -                aml_append(dev, build_q35_osc_method());
>
>
> > > +                /* Expander bridges do not have ACPI PCI Hot-plug 
> > > enabled */
> > > +                aml_append(dev, build_q35_osc_method(false));
>
> this's it not obvious, why PXBs aren't capable of ACPI PCI Hot-plug?

Lack of support from the original patchset.

> > >              } else {
> > >                  aml_append(dev, aml_name_decl("_HID", 
> > > aml_eisaid("PNP0A03")));
> > >              }
> > > --
> > > 2.31.1
> >
>