Re: [Qemu-ppc] [PATCH qemu] ppc/spapr: Receive and store device tree blob from SLOF

[Greg Kurz]

On Mon, 10 Dec 2018 17:20:43 +1100
David Gibson <address@hidden> wrote:

> On Mon, Nov 12, 2018 at 03:12:26PM +1100, Alexey Kardashevskiy wrote:
> > 
> > 
> > On 12/11/2018 05:10, Greg Kurz wrote:  
> > > Hi Alexey,
> > > 
> > > Just a few remarks. See below.
> > > 
> > > On Thu,  8 Nov 2018 12:44:06 +1100
> > > Alexey Kardashevskiy <address@hidden> wrote:
> > >   
> > >> SLOF receives a device tree and updates it with various properties
> > >> before switching to the guest kernel and QEMU is not aware of any changes
> > >> made by SLOF. Since there is no real RTAS (QEMU implements it), it makes
> > >> sense to pass the SLOF final device tree to QEMU to let it implement
> > >> RTAS related tasks better, such as PCI host bus adapter hotplug.
> > >>
> > >> Specifially, now QEMU can find out the actual XICS phandle (for PHB
> > >> hotplug) and the RTAS linux,rtas-entry/base properties (for firmware
> > >> assisted NMI - FWNMI).
> > >>
> > >> This stores the initial DT blob in the sPAPR machine and replaces it
> > >> in the KVMPPC_H_UPDATE_DT (new private hypercall) handler.
> > >>
> > >> This adds an @update_dt_enabled machine property to allow backward
> > >> migration.
> > >>
> > >> SLOF already has a hypercall since
> > >> https://github.com/aik/SLOF/commit/e6fc84652c9c0073f9183
> > >>
> > >> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> > >> ---
> > >>  include/hw/ppc/spapr.h |  7 ++++++-
> > >>  hw/ppc/spapr.c         | 29 ++++++++++++++++++++++++++++-
> > >>  hw/ppc/spapr_hcall.c   | 32 ++++++++++++++++++++++++++++++++
> > >>  hw/ppc/trace-events    |  2 ++
> > >>  4 files changed, 68 insertions(+), 2 deletions(-)
> > >>
> > >> diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
> > >> index ad4d7cfd97..f5dcaf44cb 100644
> > >> --- a/include/hw/ppc/spapr.h
> > >> +++ b/include/hw/ppc/spapr.h
> > >> @@ -100,6 +100,7 @@ struct sPAPRMachineClass {
> > >>  
> > >>      /*< public >*/
> > >>      bool dr_lmb_enabled;       /* enable dynamic-reconfig/hotplug of 
> > >> LMBs */
> > >> +    bool update_dt_enabled;    /* enable KVMPPC_H_UPDATE_DT */
> > >>      bool use_ohci_by_default;  /* use USB-OHCI instead of XHCI */
> > >>      bool pre_2_10_has_unused_icps;
> > >>      bool legacy_irq_allocation;
> > >> @@ -136,6 +137,9 @@ struct sPAPRMachineState {
> > >>      int vrma_adjust;
> > >>      ssize_t rtas_size;
> > >>      void *rtas_blob;
> > >> +    uint32_t fdt_size;
> > >> +    uint32_t fdt_initial_size;  
> > > 
> > > I don't quite see the purpose of fdt_initial_size... it seems to be only
> > > used to print a trace.  
> > 
> > 
> > Ah, lost in rebase. The purpose was to test if the new device tree has
> > not grown too much.
> > 
> > 
> >   
> > >   
> > >> +    void *fdt_blob;
> > >>      long kernel_size;
> > >>      bool kernel_le;
> > >>      uint32_t initrd_base;
> > >> @@ -462,7 +466,8 @@ struct sPAPRMachineState {
> > >>  #define KVMPPC_H_LOGICAL_MEMOP  (KVMPPC_HCALL_BASE + 0x1)
> > >>  /* Client Architecture support */
> > >>  #define KVMPPC_H_CAS            (KVMPPC_HCALL_BASE + 0x2)
> > >> -#define KVMPPC_HCALL_MAX        KVMPPC_H_CAS
> > >> +#define KVMPPC_H_UPDATE_DT      (KVMPPC_HCALL_BASE + 0x3)
> > >> +#define KVMPPC_HCALL_MAX        KVMPPC_H_UPDATE_DT
> > >>  
> > >>  typedef struct sPAPRDeviceTreeUpdateHeader {
> > >>      uint32_t version_id;
> > >> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > >> index c08130facb..5e2d4d211c 100644
> > >> --- a/hw/ppc/spapr.c
> > >> +++ b/hw/ppc/spapr.c
> > >> @@ -1633,7 +1633,10 @@ static void spapr_machine_reset(void)
> > >>      /* Load the fdt */
> > >>      qemu_fdt_dumpdtb(fdt, fdt_totalsize(fdt));
> > >>      cpu_physical_memory_write(fdt_addr, fdt, fdt_totalsize(fdt));
> > >> -    g_free(fdt);
> > >> +    g_free(spapr->fdt_blob);
> > >> +    spapr->fdt_size = fdt_totalsize(fdt);
> > >> +    spapr->fdt_initial_size = spapr->fdt_size;
> > >> +    spapr->fdt_blob = fdt;  
> > > 
> > > Hmm... It looks weird to store state in a reset handler. I'd rather zeroe
> > > both fdt_blob and fdt_size here.  
> > 
> > The device tree is built from the reset handler and the idea is that we
> > want to always have some tree in the machine.  
> 
> Yes, I think the approach here is fine.  Otherwise when we want to
> look up the current fdt state in RTAS calls or whatever we'd always
> have to do
>       if (fdt_blob)
>               look up that
>       else
>               look up qemu created fdt.
> 

No. We only have one fdt blob: the initial one, I'd rather
call reset time one, or the updated one.

> Incidentally 'fdt' and 'fdt_blob' names do a terrible job of
> distinguishing what the difference is.  Renaming fdt to fdt_initial
> (to match fdt_initial_size) and fdt_blob to fdt should make that
> clearer.
> 

As mentioned earlier in this thread, spapr->fdt_initial_size is only used
for tracing if the received fdt blob fails fdt_check_full()...

$ git grep -H fdt_initial_size
hw/ppc/spapr.c:    spapr->fdt_initial_size = spapr->fdt_size;
hw/ppc/spapr.c:        VMSTATE_UINT32(fdt_initial_size, sPAPRMachineState),
hw/ppc/spapr_hcall.c:        
trace_spapr_update_dt_failed(spapr->fdt_initial_size, cb,
include/hw/ppc/spapr.h:    uint32_t fdt_initial_size;

Not sure it is helpful, and anyway, it is expected to be the same in source
and destination, so why put it in the migration stream ?

The only case where we want to migrate something is when h_update_dt() has
succeeded, ie, the guest passed a valid DT blob. This implies that its
size isn't 0, otherwise fdt_check_full() would return -FDT_ERR_TRUNCATED.

I would suggest rather to:

- completely drop spapr->fdt_initial_size
- clear spapr->fdt_size at machine reset
- migrate if spapr->fdt_size is not zero

Also, I've just realized another problem... nothing prevents a malicious
guest to pass an insanely great size to h_update_dt, which would cause
g_malloc0() to abort... The passed size should be checked against
FDT_MAX_SIZE.

Cheers,

--
Greg

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