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


From: Michael S. Tsirkin
Subject: Re: [PATCH 4/5] hw/i386/acpi-build: Deny control on PCIe Native Hot-plug in _OSC
Date: Wed, 10 Nov 2021 10:37:15 -0500

On Wed, Nov 10, 2021 at 04:25:40PM +0100, Julia Suvorova wrote:
> 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.

Hmm. So PXB will keep using native hotplug. Looks like a good reason
to pick up Gerd's patches to fix it up ... 

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




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