Re: [Qemu-devel] [PATCH v4 11/13] pc: add 'etc/boot-cpus' fw_cfg file for machine with more than 255 CPUs

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Oct 20, 2016 at 03:27:50PM +0200, Igor Mammedov wrote:
> On Thu, 20 Oct 2016 10:27:33 -0200
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > On Thu, Oct 20, 2016 at 01:27:34PM +0200, Igor Mammedov wrote:
> > > On Wed, 19 Oct 2016 16:29:29 -0200
> > > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > >   
> > > > On Wed, Oct 19, 2016 at 05:18:38PM +0200, Igor Mammedov wrote:  
> > > > > On Wed, 19 Oct 2016 11:15:46 -0200
> > > > > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > > > >     
> > > > > > On Wed, Oct 19, 2016 at 02:05:41PM +0200, Igor Mammedov wrote:    
> > > > > > > Currently firmware uses 1 byte at 0x5F offset in RTC CMOS
> > > > > > > to get number of CPUs present at boot. However 1 byte is
> > > > > > > not enough to handle more than 255 CPUs.  So add a new
> > > > > > > fw_cfg file that would allow QEMU to tell it.
> > > > > > > For compat reasons add file only for machine types that
> > > > > > > support more than 255 CPUs.
> > > > > > > 
> > > > > > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>      
> > > > > > [...]    
> > > > > > >  static X86CPU *pc_new_cpu(const char *typename, int64_t apic_id,
> > > > > > >                            Error **errp)
> > > > > > >  {
> > > > > > > @@ -1232,6 +1221,11 @@ static void pc_build_feature_control_file(PCMachineState *pcms)
> > > > > > >      fw_cfg_add_file(pcms->fw_cfg, "etc/msr_feature_control", val, sizeof(*val));
> > > > > > >  }
> > > > > > >  
> > > > > > > +static void rtc_set_cpus_count(ISADevice *rtc, uint16_t cpus_count)
> > > > > > > +{
> > > > > > > +    rtc_set_memory(rtc, 0x5f, cpus_count - 1);      
> > > > > > 
> > > > > > If we have more than 255 CPUs, shouldn't we at least tell the old
> > > > > > BIOS that we have 255, instead of silently truncating bits?    
> > > > > It won't do any good to BIOS as it would hang in AP wakeup due to
> > > > >  (expected != woken up) condition.    
> > > > 
> > > > Even in this case, truncating bits makes it a bit unpredictable:
> > > > having 257 CPUs would set RTC memory to 0, BIOS will believe it
> > > > is a UP system.  
> > > and it will do AP wakeup regardless, where old BIOS will hang
> > > due to unexpectedly woken-up CPUs regardless of value in cmos.  
> > 
> > 0 (1 CPU) seems to be the only value that will not hang (at least
> > it won't hang at the same point). If cmos_cpu_count is 1, SeaBIOS
> > won't even wait for the other CPUs to wake up.
> I don't see it in code
> 
> here is current/old seabios smp init flow:
> 
>     if (BSP HAS APIC DISABLED) {                             
>         // No apic - only the main cpu is present.                               
>         dprintf(1, "No apic - only the main cpu is present.\n");                 
>         CountCPUs= 1;                                                            
>         return;                                                                  
>     }     
> 

We also have these lines:

    u8 apic_id = ebx>>24;
    FoundAPICIDs[apic_id/32] |= (1 << (apic_id % 32));
    CountCPUs = 1;

>     AP WAKEUP regardless of CMOS_BIOS_SMP_COUNT value
> 
>     u8 cmos_smp_count = rtc_read(CMOS_BIOS_SMP_COUNT) + 1;                       
>     while (cmos_smp_count != CountCPUs) // we are doomed here due to AP race and
>                                         // mostly hang here regardless of cmos_smp_count value
>                                         // as CountCPUs could be anything at this point and counting up

CountCPUs will be always 1 on the first (cmos_smp_count !=
CountCPUs) check, because handle_smp() (which change CountCPUs)
is protected by SMPLock.

>                                         
> 
> and it's been pretty much the same logic throughout history,
> that's why it doesn't matter  if rtc_read(CMOS_BIOS_SMP_COUNT) is 0 or something else.
> 
> SMP support has been introduced by:
> (e97ca7bd1 Forward port bochs smp changes; rename smpdetect.c to smp.c.)
> 
> 
> [...]
> > That said, setting it to 0 (1 CPU) sounds like the best option.
> > But I would be OK with any other value as long as it is
> > predictable.
> So counting above SeaBIOS behavior shall I still set cmos to 0?

In the case we get unpredictable behavior from SeaBIOS for any
value (I will make some tests to confirm that), setting it to 0
still seems more likely to avoid weird things from happening with
BIOSes or OSes we don't control.

> 
> > 
> > >   
> > > > > >     
> > > > > > > +}
> > > > > > > +
> > > > > > >  static
> > > > > > >  void pc_machine_done(Notifier *notifier, void *data)
> > > > > > >  {
> > > > > > > @@ -1240,7 +1234,7 @@ void pc_machine_done(Notifier *notifier, void *data)
> > > > > > >      PCIBus *bus = pcms->bus;
> > > > > > >  
> > > > > > >      /* set the number of CPUs */
> > > > > > > -    rtc_set_memory(pcms->rtc, 0x5f, pc_present_cpus_count(pcms) - 1);
> > > > > > > +    rtc_set_cpus_count(pcms->rtc, le16_to_cpu(pcms->boot_cpus_le));
> > > > > > >  
> > > > > > >      if (bus) {
> > > > > > >          int extra_hosts = 0;
> > > > > > > @@ -1261,8 +1255,15 @@ void pc_machine_done(Notifier *notifier, void *data)
> > > > > > >  
> > > > > > >      acpi_setup();
> > > > > > >      if (pcms->fw_cfg) {
> > > > > > > +        MachineClass *mc = MACHINE_GET_CLASS(pcms);
> > > > > > > +
> > > > > > >          pc_build_smbios(pcms->fw_cfg);
> > > > > > >          pc_build_feature_control_file(pcms);
> > > > > > > +
> > > > > > > +        if (mc->max_cpus > 255) {
> > > > > > > +            fw_cfg_add_file(pcms->fw_cfg, "etc/boot-cpus", &pcms->boot_cpus_le,
> > > > > > > +                            sizeof(pcms->boot_cpus_le));
> > > > > > > +        }
> > > > > > >      }
> > > > > > >  }
> > > > > > >  
> > > > > > > @@ -1786,9 +1787,11 @@ static void pc_cpu_plug(HotplugHandler *hotplug_dev,
> > > > > > >          }
> > > > > > >      }
> > > > > > >  
> > > > > > > +    /* increment the number of CPUs */
> > > > > > > +    pcms->boot_cpus_le = cpu_to_le16(le16_to_cpu(pcms->boot_cpus_le) + 1);      
> > > > > > 
> > > > > > Is this really safe? What if the guest is in the middle of a
> > > > > > etc/boot-cpus read?    
> > > > > It's safe for boot CPUs but
> > > > > it's not safe to hotplug cpus CPU during BIOS boot at all
> > > > > as number of CPUs read from boot_cpus might not match number
> > > > > of CPUs that received INIT/SIPI wakeup.
> > > > > This problem is ignored for now, I've dropped related SeaBIOS patch
> > > > > by Kevin's request and would explore it some more to avoid race there.
> > > > > 
> > > > > Anyways,
> > > > > Do you have an idea how to improve reading from pcms->boot_cpus_le and make it atomic?    
> > > > 
> > > > No idea. SeaBIOS seems to use insb to read fw_cfg files, so we
> > > > could be updating the data between two reads. I don't think we
> > > > want to design a fw_cfg guest<->host synchronization mechanism.
> > > > 
> > > > I believe the solution is to not change it at all after booting
> > > > the guest. I suggest initializing/reinitializing fw_cfg data only
> > > > on reset.  
> > > I've checked it once more and value read by BIOS atomically
> > > as both QEMU and SeaBIOS use dma interface to transfer data.
> > > BIOS transfers control to QEMU once via
> > > 
> > >  outl(PORT_QEMU_CFG_DMA_ADDR_LOW)
> > > 
> > > and after that access to pcms->boot_cpus_le is protected by BQL.
> > > So there is no need to invent some sort of synchronization
> > > or limit update to fixed points (machine_done/reset).  
> > 
> > You're right. I was assuming the worst case and looking at the
> > non-DMA path.
> > 
> > In this case, I believe we're safe except when running old BIOS
> > that doesn't support fw_cfg DMA.
> old bios won't read this file (as it doesn't know about it)

Oops, you're right. :)

-- 
Eduardo