On Thu, Oct 20, 2016 at 03:27:50PM +0200, Igor Mammedov wrote:
> On Thu, 20 Oct 2016 10:27:33 -0200
> Eduardo Habkost <ehabk...@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 <ehabk...@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 <ehabk...@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 <imamm...@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
