Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type

[Michael S. Tsirkin]

On Wed, Jul 24, 2019 at 01:14:35PM +0200, Paolo Bonzini wrote:
> On 23/07/19 12:01, Paolo Bonzini wrote:
> > The number of buses is determined by the firmware, not by QEMU, so
> > fw_cfg would not be the right interface.  In fact (as I have just
> > learnt) lastbus is an x86-specific option that overrides the last bus
> > returned by SeaBIOS's handle_1ab101.
> > 
> > So the next step could be to figure out what is the lastbus returned by
> > handle_1ab101 and possibly why it isn't zero.
> 
> Some update:
> 
> - for 64-bit, PCIBIOS (and thus handle_1ab101) is not called.  PCIBIOS is
> only used by 32-bit kernels.  As a side effect, PCI expander bridges do not
> work on 32-bit kernels with ACPI disabled, because they are located beyond
> pcibios_last_bus (with ACPI enabled, the DSDT exposes them).
> 
> - for -M pc, pcibios_last_bus in Linux remains -1 and no "legacy scanning" is 
> done.
> 
> - for -M q35, pcibios_last_bus in Linux is set based on the size of the 
> MMCONFIG aperture and Linux ends up scanning all 32*255 (bus,dev) pairs 
> for buses above 0.
> 
> Here is a patch that only scans devfn==0, which should mostly remove the need
> for pci=lastbus=0.  (Testing is welcome).

Actually, I think I have a better idea.
At the moment we just get an exit on these reads and return all-ones.
Yes, in theory there could be a UR bit set in a bunch of
registers but in practice no one cares about these,
and I don't think we implement them.
So how about mapping a single page, read-only, and filling it
with all-ones?

We'll still run the code within linux but it will be free.

What do you think?


> Actually, KVM could probably avoid the scanning altogether.  The only 
> "hidden" root
> buses we expect are from PCI expander bridges and if you found an MMCONFIG 
> area
> through the ACPI MCFG table, you can also use the DSDT to find PCI expander 
> bridges.
> However, I am being conservative.
> 
> A possible alternative could be a mechanism whereby the vmlinuz real mode 
> entry
> point, or the 32-bit PVH entry point, fetch lastbus and they pass it to the
> kernel via the vmlinuz or PVH boot information structs.  However, I don't 
> think
> that's very useful, and there is some risk of breaking real hardware too.
> 
> Paolo
> 
> diff --git a/arch/x86/include/asm/pci_x86.h b/arch/x86/include/asm/pci_x86.h
> index 73bb404f4d2a..17012aa60d22 100644
> --- a/arch/x86/include/asm/pci_x86.h
> +++ b/arch/x86/include/asm/pci_x86.h
> @@ -61,6 +61,7 @@ enum pci_bf_sort_state {
>  extern struct pci_ops pci_root_ops;
>  
>  void pcibios_scan_specific_bus(int busn);
> +void pcibios_scan_bus_by_device(int busn);
>  
>  /* pci-irq.c */
>  
> @@ -216,8 +217,10 @@ static inline void mmio_config_writel(void __iomem *pos, 
> u32 val)
>  # endif
>  # define x86_default_pci_init_irq    pcibios_irq_init
>  # define x86_default_pci_fixup_irqs  pcibios_fixup_irqs
> +# define x86_default_pci_scan_bus    pcibios_scan_bus_by_device
>  #else
>  # define x86_default_pci_init                NULL
>  # define x86_default_pci_init_irq    NULL
>  # define x86_default_pci_fixup_irqs  NULL
> +# define x86_default_pci_scan_bus      NULL
>  #endif
> diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
> index b85a7c54c6a1..4c3a0a17a600 100644
> --- a/arch/x86/include/asm/x86_init.h
> +++ b/arch/x86/include/asm/x86_init.h
> @@ -251,6 +251,7 @@ struct x86_hyper_runtime {
>   * @save_sched_clock_state:  save state for sched_clock() on suspend
>   * @restore_sched_clock_state:       restore state for sched_clock() on 
> resume
>   * @apic_post_init:          adjust apic if needed
> + * @pci_scan_bus:            scan a PCI bus
>   * @legacy:                  legacy features
>   * @set_legacy_features:     override legacy features. Use of this callback
>   *                           is highly discouraged. You should only need
> @@ -273,6 +274,7 @@ struct x86_platform_ops {
>       void (*save_sched_clock_state)(void);
>       void (*restore_sched_clock_state)(void);
>       void (*apic_post_init)(void);
> +     void (*pci_scan_bus)(int busn);
>       struct x86_legacy_features legacy;
>       void (*set_legacy_features)(void);
>       struct x86_hyper_runtime hyper;
> diff --git a/arch/x86/kernel/jailhouse.c b/arch/x86/kernel/jailhouse.c
> index 6857b4577f17..b248d7036dd3 100644
> --- a/arch/x86/kernel/jailhouse.c
> +++ b/arch/x86/kernel/jailhouse.c
> @@ -11,12 +11,14 @@
>  #include <linux/acpi_pmtmr.h>
>  #include <linux/kernel.h>
>  #include <linux/reboot.h>
> +#include <linux/pci.h>
>  #include <asm/apic.h>
>  #include <asm/cpu.h>
>  #include <asm/hypervisor.h>
>  #include <asm/i8259.h>
>  #include <asm/irqdomain.h>
>  #include <asm/pci_x86.h>
> +#include <asm/pci.h>
>  #include <asm/reboot.h>
>  #include <asm/setup.h>
>  #include <asm/jailhouse_para.h>
> @@ -136,6 +138,22 @@ static int __init jailhouse_pci_arch_init(void)
>       return 0;
>  }
>  
> +static void jailhouse_pci_scan_bus_by_function(int busn)
> +{
> +        int devfn;
> +        u32 l;
> +
> +        for (devfn = 0; devfn < 256; devfn++) {
> +                if (!raw_pci_read(0, busn, devfn, PCI_VENDOR_ID, 2, &l) &&
> +                    l != 0x0000 && l != 0xffff) {
> +                        DBG("Found device at %02x:%02x [%04x]\n", busn, 
> devfn, l);
> +                        pr_info("PCI: Discovered peer bus %02x\n", busn);
> +                        pcibios_scan_root(busn);
> +                        return;
> +                }
> +        }
> +}
> +
>  static void __init jailhouse_init_platform(void)
>  {
>       u64 pa_data = boot_params.hdr.setup_data;
> @@ -153,6 +171,7 @@ static void __init jailhouse_init_platform(void)
>       x86_platform.legacy.rtc         = 0;
>       x86_platform.legacy.warm_reset  = 0;
>       x86_platform.legacy.i8042       = X86_LEGACY_I8042_PLATFORM_ABSENT;
> +     x86_platform.pci_scan_bus       = jailhouse_pci_scan_bus_by_function;
>  
>       legacy_pic                      = &null_legacy_pic;
>  
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 82caf01b63dd..59f7204ed8f3 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -24,6 +24,7 @@
>  #include <linux/debugfs.h>
>  #include <linux/nmi.h>
>  #include <linux/swait.h>
> +#include <linux/pci.h>
>  #include <asm/timer.h>
>  #include <asm/cpu.h>
>  #include <asm/traps.h>
> @@ -33,6 +34,7 @@
>  #include <asm/apicdef.h>
>  #include <asm/hypervisor.h>
>  #include <asm/tlb.h>
> +#include <asm/pci.h>
>  
>  static int kvmapf = 1;
>  
> @@ -621,10 +623,31 @@ static void kvm_flush_tlb_others(const struct cpumask 
> *cpumask,
>       native_flush_tlb_others(flushmask, info);
>  }
>  
> +#ifdef CONFIG_PCI
> +static void kvm_pci_scan_bus(int busn)
> +{
> +        u32 l;
> +
> +     /*
> +      * Assume that there are no "hidden" buses, i.e. all PCI root buses
> +      * have a host bridge at device 0, function 0.
> +      */
> +     if (!raw_pci_read(0, busn, 0, PCI_VENDOR_ID, 2, &l) &&
> +         l != 0x0000 && l != 0xffff) {
> +             pr_info("PCI: Discovered peer bus %02x\n", busn);
> +             pcibios_scan_root(busn);
> +        }
> +}
> +#endif
> +
>  static void __init kvm_guest_init(void)
>  {
>       int i;
>  
> +#ifdef CONFIG_PCI
> +     x86_platform.pci_scan_bus = kvm_pci_scan_bus;
> +#endif
> +
>       if (!kvm_para_available())
>               return;
>  
> diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
> index 50a2b492fdd6..19e1cc2cb6e0 100644
> --- a/arch/x86/kernel/x86_init.c
> +++ b/arch/x86/kernel/x86_init.c
> @@ -118,6 +118,7 @@ struct x86_platform_ops x86_platform __ro_after_init = {
>       .get_nmi_reason                 = default_get_nmi_reason,
>       .save_sched_clock_state         = tsc_save_sched_clock_state,
>       .restore_sched_clock_state      = tsc_restore_sched_clock_state,
> +     .pci_scan_bus                   = x86_default_pci_scan_bus,
>       .hyper.pin_vcpu                 = x86_op_int_noop,
>  };
>  
> diff --git a/arch/x86/pci/legacy.c b/arch/x86/pci/legacy.c
> index 467311b1eeea..6214dbce26d3 100644
> --- a/arch/x86/pci/legacy.c
> +++ b/arch/x86/pci/legacy.c
> @@ -36,14 +36,19 @@ int __init pci_legacy_init(void)
>  
>  void pcibios_scan_specific_bus(int busn)
>  {
> -     int stride = jailhouse_paravirt() ? 1 : 8;
> -     int devfn;
> -     u32 l;
> -
>       if (pci_find_bus(0, busn))
>               return;
>  
> -     for (devfn = 0; devfn < 256; devfn += stride) {
> +     x86_platform.pci_scan_bus(busn);
> +}
> +EXPORT_SYMBOL_GPL(pcibios_scan_specific_bus);
> +
> +void pcibios_scan_bus_by_device(int busn)
> +{
> +     int devfn;
> +     u32 l;
> +
> +     for (devfn = 0; devfn < 256; devfn += 8) {
>               if (!raw_pci_read(0, busn, devfn, PCI_VENDOR_ID, 2, &l) &&
>                   l != 0x0000 && l != 0xffff) {
>                       DBG("Found device at %02x:%02x [%04x]\n", busn, devfn, 
> l);
> @@ -53,7 +58,6 @@ void pcibios_scan_specific_bus(int busn)
>               }
>       }
>  }
> -EXPORT_SYMBOL_GPL(pcibios_scan_specific_bus);
>  
>  static int __init pci_subsys_init(void)
>  {