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

[Igor Mammedov]

On Thu, 20 Oct 2016 10:27:33 -0200
Eduardo Habkost <address@hidden> 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 <address@hidden> 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 <address@hidden> 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 <address@hidden>      
> > > > > [...]    
> > > > > >  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;                                                                 
 
    }     

    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
                                        

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?

> 
> >   
> > > > >     
> > > > > > +}
> > > > > > +
> > > > > >  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)


> 
> >   
> > > After that, we could block or delay CPU hotplug until we know the
> > > guest will be able to handle it. Do we have anything that
> > > prevents or delays hotplug until we know the guest is able to
> > > handle the hotplug events?  
> > Not that I know of,
> > and I'd like to fix BIOS side to handle AP wakeup gracefully
> > even if cpus are hotplugged while BIOS is running instead of
> > putting band-aids to workaround the issue.
> >   
> 
> No problem to me, as the DMA path seems safe.
>