Re: [Qemu-devel] [Bug 1297651] [NEW] KVM create a win7 guest with Qemu, it boots up fail

[Igor Mammedov]

On Wed, 26 Mar 2014 14:58:28 +0200
"Michael S. Tsirkin" <address@hidden> wrote:

> On Wed, Mar 26, 2014 at 01:28:02PM +0100, Laszlo Ersek wrote:
> > On 03/26/14 11:31, Michael S. Tsirkin wrote:
> > 
> > > On Wed, Mar 26, 2014 at 06:45:10AM -0000, Robert Hu wrote:
> > 
> > >> Date:   Mon Mar 17 17:05:16 2014 +0100
> > >>
> > >>     i386/acpi-build: allow more than 255 elements in CPON
> > >>
> > >>     The build_ssdt() function builds a number of AML objects that are 
> > >> related
> > >>     to CPU hotplug, and whose IDs form a contiguous sequence of APIC IDs.
> > >>     (APIC IDs are in fact discontiguous, but this is the traditional
> > >>     interface: build a contiguous sequence from zero up that covers all
> > >>     possible APIC IDs.) These objects are:
> > >>
> > >>     - a Processor() object for each VCPU,
> > >>     - a NTFY method, with one branch for each VCPU,
> > >>     - a CPON package with one element (hotplug status byte) for each 
> > >> VCPU.
> > >>
> > >>     The build_ssdt() function currently limits the *count* of processor
> > >>     objects, and NTFY branches, and CPON elements, in 0xFF (see the 
> > >> assignment
> > >>     to "acpi_cpus"). This allows for an inclusive APIC ID range of 
> > >> [0..254].
> > >>     This is incorrect, because the highest APIC ID that we otherwise 
> > >> allow a
> > >>     VCPU to take is 255.
> > >>
> > >>     In order to extend the maximum count to 256, and the traversed APIC 
> > >> ID
> > >>     range correspondingly to [0..255]:
> > >>     - the Processor() objects need no change,
> > >>     - the NTFY method also needs no change,
> > >>     - the CPON package must be updated, because it is defined with a
> > >>       DefPackage, and the number of elements in such a package can be at 
> > >> most
> > >>       255. We pick a DefVarPackage instead.
> > >>
> > >>     We replace the Op byte, and the encoding of the number of elements.
> > >>     Compare:
> > >>
> > >>     DefPackage     := PackageOp    PkgLength NumElements    
> > >> PackageElementList
> > >>     DefVarPackage  := VarPackageOp PkgLength VarNumElements 
> > >> PackageElementList
> > >>
> > >>     PackageOp      := 0x12
> > >>     VarPackageOp   := 0x13
> > > 
> > > 
> > > I think I know what's going on here: the specification says:
> > > 
> > > The ASL compiler can emit two different AML opcodes for a Package
> > > declaration, either PackageOp or VarPackageOp. For small, fixed-length
> > > packages, the PackageOp is used and this
> > > 
> > > opcode is compatible with ACPI 1.0. A VarPackageOp will be emitted if
> > > any of the following conditions are true:
> > > •
> > >  The NumElements argument is a TermArg that can only be resolved at
> > > runtime.
> > > •
> > >  At compile time, NumElements resolves to a constant that is larger than
> > > 255.
> > > •
> > >  The PackageList contains more than 255 initializer elements.
> > > 
> > > 
> > > So we clearly violate this rule.
> > 
> > I did see this passage of the spec, but it is not relevant. It is about 
> > what the ASL compiler does. It comes from:
> > 
> > 19      ACPI Source Language (ASL)Reference
> > 19.5    ASL Operator Reference
> > 19.5.98 Package (Declare Package Object)
> > 
> > We do not have an ASL compiler at hand.
> 
> True. But I think the spec and guests simply didn't envision writing AML by 
> hand :)
> 
> > The specification nowhere restricts VarPackageOp to > 255.
> > 
> > However, what I *did* mess up is compatibility with ACPI 1.0. Just below 
> > the quoted part, there's also this:
> > 
> >   Note: The ability to create variable-sized packages was first
> >         introduced in ACPI 2.0. ACPI 1.0 only allowed fixed-size
> >         packages with up to 255 elements.
> > 
> > I forgot that the header of the containing table stated the ACPI version:
> > 
> >     /* Copy header and patch values in the S3_ / S4_ / S5_ packages */
> >     ssdt_ptr = acpi_data_push(table_data, sizeof(ssdp_misc_aml));
> > 
> > and
> > 
> >   DefinitionBlock ("ssdt-misc.aml",  "SSDT", 0x01, "BXPC", "BXSSDTSUSP", 
> > 0x1)
> >                                              ^^^^
> >                                         ComplianceRevision
> > 
> > So my patch generates ACPI 2.0+ contents for an 1.0 table.
> > 
> > > The following seems to fix the issue - still testing. Can you confirm 
> > > please?
> > 
> > This patch only restricts the bug to a subset of cases, but it doesn't fix 
> > it.
> > 
> > > However the question we should ask is whether
> > > it's a good idea to allow hotplug ID values that might
> > > make guests fail to boot.
> > > 
> > > How about limiting ACPI_CPU_HOTPLUG_ID_LIMIT to 255?
> > 
> > I think it's not the package size / APIC ID value per se that breaks the 
> > boot, but the incompatibility between the ACPI revision stated in the SSDT 
> > header, and the construct in the table.
> 
> 
> It would be interesting to try tweaking the table version and seeing
> what happens. Does it help any guests?
It won't help XP based guests since they support 1.0 revision only.

> 
> > > 
> > > We never allowed > 255 in the past, is it worth the
> > > maintainance headaches?
> > > 
> > >  
> > > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> > > index f1054dd..7597517 100644
> > > --- a/hw/i386/acpi-build.c
> > > +++ b/hw/i386/acpi-build.c
> > > @@ -1055,9 +1055,21 @@ build_ssdt(GArray *table_data, GArray *linker,
> > >  
> > >          {
> > >              GArray *package = build_alloc_array();
> > > -            uint8_t op = 0x13; /* VarPackageOp */
> > > +            uint8_t op;
> > > +
> > > +            /*
> > > +             * Note: The ability to create variable-sized packages was 
> > > first introduced in ACPI 2.0. ACPI 1.0 only
> > > +             * allowed fixed-size packages with up to 255 elements.
> > > +             * Windows guests up to win2k8 fail when VarPackageOp is 
> > > used.
> > > +             */
> > > +            if (acpi_cpus <= 255) {
> > > +                op = 0x12; /* PackageOp */
> > > +                build_append_byte(package, acpi_cpus); /* NumElements */
> > > +            } else {
> > > +                op = 0x13; /* VarPackageOp */
> > > +                build_append_int(package, acpi_cpus); /* VarNumElements 
> > > */
> > > +            }
> > >  
> > > -            build_append_int(package, acpi_cpus); /* VarNumElements */
> > >              for (i = 0; i < acpi_cpus; i++) {
> > >                  uint8_t b = test_bit(i, cpu->found_cpus) ? 0x01 : 0x00;
> > >                  build_append_byte(package, b);
> > > 
> > 
> > The patch will mask the problem for most of the cases, but when 
> > VarPackageOp is used, it will be broken just the same (because the ACPI 
> > revision in the SSDT header will still mismatch).
> 
> yes but modern guests don't seem to care, and it was already broken in
> 1.7, wasn't it?
> 
> > Here's my proposal:
> > - I can post a patch that changes the SSDT DSL files, *and* the DSDT files 
> > (q35-acpi-dsdt.dsl acpi-dsdt.dsl), to state an ACPI revision of 2.0. (The 
> > ACPI revision of the DSDT file determines integer sizes for SSDTs too, so 
> > we can't just bump the SSDTs to 2.0)
> 
> It should not be a problem but I'm not sure I get this comment. Can you 
> explain?
> 
> > - Or I suggest to revert my patches for 2.0.
> > 
> > You probably won't like bumping the ACPI rev in DSDT/SSDT, for various 
> > compatibility reasons, so I think I suggest to revert these two patches of 
> > mine. It's now clear to me that this VCPU hotplug limit cannot be lifted 
> > without much more intrusive (and guest visible) changes. Sorry about 
> > missing this fact in my original submission.
> > 
> > Thanks,
> > Laszlo
> 
> I have a problem with both approaches :)
> 
> If we want to change ACPI rev, I think we should do this
> conditionally when max_cpus > 255.
> Would be worth it if this fixes some guests.
> 
> As for reverting, I think it's a problem that we seem to
> allow max_cpus = 256 but then it doesn't really work.
more clean would be to abort if CPON index (i.e. APIC ID)
is more than 255. That would affect small number of weird
topologies but sould be fine for most usecases.

> 
> 
> 
> I think the patch I posted might be good enough for 2.0.
> It seems to make things work for new guests, and old guests
> work as long as you don't specify max_cpus = 255.
> The config with a high max_cpus value never really worked so
> not a big deal.
> 
> 
> Alternatively limit max_cpus to 255, to make it fail cleanly.
that won't work since max_cpus is not equivalent to index in CPON
which depends on topology as well.