Re: [Qemu-devel] [PATCH 6/7] target-i386: add implementation of query-cpudefs

[Eduardo Habkost]

On Fri, Aug 10, 2012 at 11:37:30AM -0500, Anthony Liguori wrote:
> Eduardo Habkost <address@hidden> writes:
> 
> > On Fri, Aug 10, 2012 at 09:43:21AM -0500, Anthony Liguori wrote:
> >> Eduardo Habkost <address@hidden> writes:
> >> 
> >> > On Fri, Jul 27, 2012 at 08:37:18AM -0500, Anthony Liguori wrote:
> >> >> Signed-off-by: Anthony Liguori <address@hidden>
> >> >> ---
> >> >>  target-i386/cpu.c |   22 ++++++++++++++++++++++
> >> >>  1 files changed, 22 insertions(+), 0 deletions(-)
> >> >> 
> >> >> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> >> >> index 6b9659f..b398439 100644
> >> >> --- a/target-i386/cpu.c
> >> >> +++ b/target-i386/cpu.c
> >> >> @@ -28,6 +28,7 @@
> >> >>  #include "qemu-config.h"
> >> >>  
> >> >>  #include "qapi/qapi-visit-core.h"
> >> >> +#include "qmp-commands.h"
> >> >>  
> >> >>  #include "hyperv.h"
> >> >>  
> >> >> @@ -1123,6 +1124,27 @@ void x86_cpu_list(FILE *f, fprintf_function 
> >> >> cpu_fprintf, const char *optarg)
> >> >>      }
> >> >>  }
> >> >>  
> >> >> +CpuDefInfoList *qmp_query_cpudefs(Error **errp)
> >> >> +{
> >> >> +    CpuDefInfoList *cpu_list = NULL;
> >> >> +    x86_def_t *def;
> >> >> +
> >> >> +    for (def = x86_defs; def; def = def->next) {
> >> >> +        CpuDefInfoList *entry;
> >> >> +        CpuDefInfo *info;
> >> >> +
> >> >> +        info = g_malloc0(sizeof(*info));
> >> >> +        info->name = g_strdup(def->name);
> >> >> +
> >> >> +        entry = g_malloc0(sizeof(*entry));
> >> >> +        entry->value = info;
> >> >> +        entry->next = cpu_list;
> >> >> +        cpu_list = entry;
> >> >> +    }
> >> >> +
> >> >> +    return cpu_list;
> >> >> +}
> >> >
> >> > How would the interface look like once we:
> >> > - let libvirt know which features are available on each CPU model
> >> >   (libvirt needs that information[1]); and
> >> 
> >> I'm not sure I understand why libvirt needs this information.  Can you 
> >> elaborate?
> >
> > I see two reasons:
> >
> > - The libvirt API has functions to tell the user which features are
> >   going to be enabled for each CPU model, so it needs to know which
> >   features are enabled or not, for each machine-type + cpu-model
> >   combination, so this information can be reported proeprly.
> 
> Ok, step number one is that CPU 'features' need to be defined more
> formally.  By formally, I mean via qapi-schema.json.
> 
> Then we can extend this command to return the set of features supported
> by each CPU type.
> 
> The first step will need to sort out how this maps across architectures.
> 
> >   - Also, if libvirt can enable/disable specific CPU features in the
> >     command-line, it just makes sens to know which ones are already
> >     enabled in each built-in CPU model.
> >
> > - Probing for migration: libvirt needs to know if a given CPU model on a
> >   host can be migrated to another host. To know that, two pieces of
> >   information are needed:
> >   A) Which CPU features are visible to the guest for a specific
> >      configuration;
> >   B) Which of those features are really supported by the host
> >      hardware+kernel+QEMU, on the destination host, so it can
> >      know if migration is really possible.
> 
> Note that what QEMU thinks it exposes is not necessarily what gets
> exposed.  KVM may mask additional features.  How is this handled today?

No, what QEMU thinks it exposes actually is what gets exposed (and if it
is not, it's a bug we have to fix). This is handled using the KVM
GET_SUPPORTED_CPUID ioctl().


> 
> >> > - add machine-type-specific cpudef compatibility changes?
> >> 
> >> I think we've discussed this in IRC.  I don't think we need to worry
> >> about this.
> >
> > I remember discussing a lot about the mechanism we will use to add the
> > compatibility changes, but I don t know how the query API will look
> > like, after we implement this mechanism.
> 
> 0) User-defined CPU definitions go away
>    - We already made a big step in this direction
> 
> 1) CPU becomes a DeviceState

1.1) CPU models become classes

> 
> 2) Features are expressed as properties
> 
> 3) Same global mechanism used for everything else is used for CPUs

This is basically the compatibility mechanism we agreed upon, yes, but
what about the probing mechanism to allow libvirt to know what will be
the result of "-machine M -cpu C"[1] before actually starting a VM?

[1] By "result" I mean:
   - Whether that combination can be run properly on that host;
   - Which CPU features will be visible to the guest in case it runs.
   Both items depend on CPU model _and_ machine-type, that's why we need
   some probing mechanism that depends on the machine-type or use the
   machine-type as input.


> 
> Regards,
> 
> Anthony Liguori
> 
> >> > Would the command report different results depending on -machine?
> >> 
> >> No.
> >
> > The problem is:
> >
> > 1) We need to introduce fixes on a CPU model that changes the set of
> >    guest-visible features (add or remove a feature)[1];
> > 2) The fix has to keep compatibility, so older machine-types will
> >    keep exposing the old set of gues-visible features;
> >    - That means different machine-types will have different CPU
> >      features being exposed.
> > 3) libvirt needs to control/know which guest-visible CPU features are
> >    available to the guest (see above);
> > 4) Because of (2), the querying system used by libvirt need to depend on
> >    the CPU model and machine-type.
> >
> >
> > [1] Example:
> >     The SandyBridge model today has the "tsc-deadline" bit set, but
> >     QEMU-1.1 did not expose the tsc-deadline feature properly because of
> >     incorrect expectations about the GET_SUPPORTED_CPUID ioctl. This was
> >     fixed on qemu-1.2.
> >     
> >     That means "qemu-1.1 -machine pc-1.1 -cpu SandyBridge" does _not_
> >     expose tsc-deadline to the guest, and we need to make "qemu-1.2
> >     -machine pc-1.1 -cpu SandyBridge" _not_ expose it, too (otherwise
> >     migration from qemu-1.1 to qemu-1.2 will be broken).
> >
> >> 
> >> >
> >> > Would the command return the latest cpudef without any machine-type
> >> > hacks, and libvirt would have to query for the cpudef compatibility data
> >> > for each machine-type and combine both pieces of information itself?
> >> 
> >> I'm not sure what you mean by compatibility data.
> >
> > I mean any guest-visible compatibility bit that we will need to
> > introduce on older machine-types, when making changes on CPU models (see
> > the SandyBridge + tsc-deadline example above).
> >
> > I see two options:
> > - Libvirt queries for a [f(machine_type, cpu_model) -> cpu_features]
> >   function, that will take into account the machine-type-specific
> >   compatibility bits.
> > - Libvirt queries for a [f(cpu_model) -> cpu_features] function and a
> >   [f(machine_type) -> compatibility_changes] function, and combine both.
> >   - I don't like this approach, I am just including it as a possible
> >     alternative.
> >
> >> 
> >> Regards,
> >> 
> >> Anthony Liguori
> >> 
> >> >
> >> > [1] Note that it doesn't have to be low-level leaf-by-leaf
> >> >     register-by-register CPUID bits (I prefer a more high-level
> >> >     interface, myself), but it has to at least say "feature FOO is
> >> >     enabled/disabled" for a set of features libvirt cares about.
> >> >
> >> > -- 
> >> > Eduardo
> >> 
> >
> > -- 
> > Eduardo
> 

-- 
Eduardo