Re: [Qemu-ppc] [Qemu-devel] [PATCHv2 for-2.10 2/5] pseries: Implement HPT resizing

[David Gibson]

On Wed, Mar 15, 2017 at 12:44:18PM +0530, Bharata B Rao wrote:
> On Tue, Mar 14, 2017 at 11:04 AM, David Gibson <address@hidden>
> wrote:
> 
> > This patch implements hypercalls allowing a PAPR guest to resize its own
> > hash page table.  This will eventually allow for more flexible memory
> > hotplug.
> >
> > The implementation is partially asynchronous, handled in a special thread
> > running the hpt_prepare_thread() function.  The state of a pending resize
> > is stored in SPAPR_MACHINE->pending_hpt.
> >
> > The H_RESIZE_HPT_PREPARE hypercall will kick off creation of a new HPT, or,
> > if one is already in progress, monitor it for completion.  If there is an
> > existing HPT resize in progress that doesn't match the size specified in
> > the call, it will cancel it, replacing it with a new one matching the
> > given size.
> >
> > The H_RESIZE_HPT_COMMIT completes transition to a resized HPT, and can only
> > be called successfully once H_RESIZE_HPT_PREPARE has successfully
> > completed initialization of a new HPT.  The guest must ensure that there
> > are no concurrent accesses to the existing HPT while this is called (this
> > effectively means stop_machine() for Linux guests).
> >
> > For now H_RESIZE_HPT_COMMIT goes through the whole old HPT, rehashing each
> > HPTE into the new HPT.  This can have quite high latency, but it seems to
> > be of the order of typical migration downtime latencies for HPTs of size
> > up to ~2GiB (which would be used in a 256GiB guest).
> >
> > In future we probably want to move more of the rehashing to the "prepare"
> > phase, by having H_ENTER and other hcalls update both current and
> > pending HPTs.  That's a project for another day, but should be possible
> > without any changes to the guest interface.
> >
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> >  hw/ppc/spapr.c          |   4 +-
> >  hw/ppc/spapr_hcall.c    | 306 ++++++++++++++++++++++++++++++
> > +++++++++++++++++-
> >  include/hw/ppc/spapr.h  |   6 +
> >  target/ppc/mmu-hash64.h |   4 +
> >  4 files changed, 314 insertions(+), 6 deletions(-)
> >
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 558109c..83db110 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -94,8 +94,6 @@
> >
> >  #define PHANDLE_XICP            0x00001111
> >
> > -#define HTAB_SIZE(spapr)        (1ULL << ((spapr)->htab_shift))
> > -
> >  static int try_create_xics(sPAPRMachineState *spapr, const char
> > *type_ics,
> >                             const char *type_icp, int nr_servers,
> >                             int nr_irqs, Error **errp)
> > @@ -1169,7 +1167,7 @@ static void spapr_store_hpte(PPCVirtualHypervisor
> > *vhyp, hwaddr ptex,
> >      }
> >  }
> >
> > -static int spapr_hpt_shift_for_ramsize(uint64_t ramsize)
> > +int spapr_hpt_shift_for_ramsize(uint64_t ramsize)
> >  {
> >      int shift;
> >
> > diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> > index 9f88960..cdafc3f 100644
> > --- a/hw/ppc/spapr_hcall.c
> > +++ b/hw/ppc/spapr_hcall.c
> > @@ -3,6 +3,7 @@
> >  #include "sysemu/hw_accel.h"
> >  #include "sysemu/sysemu.h"
> >  #include "qemu/log.h"
> > +#include "qemu/error-report.h"
> >  #include "cpu.h"
> >  #include "exec/exec-all.h"
> >  #include "helper_regs.h"
> > @@ -352,20 +353,286 @@ static target_ulong h_read(PowerPCCPU *cpu,
> > sPAPRMachineState *spapr,
> >      return H_SUCCESS;
> >  }
> >
> > +struct sPAPRPendingHPT {
> > +    /* These fields are read-only after initialization */
> > +    int shift;
> > +    QemuThread thread;
> > +
> > +    /* These fields are protected by the BQL */
> > +    bool complete;
> > +
> > +    /* These fields are private to the preparation thread if
> > +     * !complete, otherwise protected by the BQL */
> > +    int ret;
> > +    void *hpt;
> > +};
> > +
> > +static void free_pending_hpt(sPAPRPendingHPT *pending)
> > +{
> > +    if (pending->hpt) {
> > +        qemu_vfree(pending->hpt);
> > +    }
> > +
> > +    g_free(pending);
> > +}
> > +
> > +static void *hpt_prepare_thread(void *opaque)
> > +{
> > +    sPAPRPendingHPT *pending = opaque;
> > +    size_t size = 1ULL << pending->shift;
> > +
> > +    pending->hpt = qemu_memalign(size, size);
> > +    if (pending->hpt) {
> > +        memset(pending->hpt, 0, size);
> > +        pending->ret = H_SUCCESS;
> > +    } else {
> > +        pending->ret = H_NO_MEM;
> > +    }
> > +
> > +    qemu_mutex_lock_iothread();
> > +
> > +    if (SPAPR_MACHINE(qdev_get_machine())->pending_hpt == pending) {
> > +        /* Ready to go */
> > +        pending->complete = true;
> > +    } else {
> > +        /* We've been cancelled, clean ourselves up */
> > +        free_pending_hpt(pending);
> > +    }
> > +
> > +    qemu_mutex_unlock_iothread();
> > +    return NULL;
> > +}
> > +
> > +/* Must be called with BQL held */
> > +static void cancel_hpt_prepare(sPAPRMachineState *spapr)
> > +{
> > +    sPAPRPendingHPT *pending = spapr->pending_hpt;
> > +
> > +    /* Let the thread know it's cancelled */
> > +    spapr->pending_hpt = NULL;
> > +
> > +    if (!pending) {
> > +        /* Nothing to do */
> > +        return;
> > +    }
> > +
> > +    if (!pending->complete) {
> > +        /* thread will clean itself up */
> > +        return;
> > +    }
> > +
> > +    free_pending_hpt(pending);
> > +}
> > +
> >  static target_ulong h_resize_hpt_prepare(PowerPCCPU *cpu,
> >                                           sPAPRMachineState *spapr,
> >                                           target_ulong opcode,
> >                                           target_ulong *args)
> >  {
> >      target_ulong flags = args[0];
> > -    target_ulong shift = args[1];
> > +    int shift = args[1];
> > +    sPAPRPendingHPT *pending = spapr->pending_hpt;
> > +    uint64_t current_ram_size;
> >
> >      if (spapr->resize_hpt == SPAPR_RESIZE_HPT_DISABLED) {
> >          return H_AUTHORITY;
> >      }
> >
> >      trace_spapr_h_resize_hpt_prepare(flags, shift);
> > -    return H_HARDWARE;
> > +
> > +    if (flags != 0) {
> > +        return H_PARAMETER;
> > +    }
> > +
> > +    if (shift && ((shift < 18) || (shift > 46))) {
> > +        return H_PARAMETER;
> > +    }
> > +
> > +    current_ram_size = pc_existing_dimms_capacity(&error_fatal);
> > +
> > +    /* We only allow the guest to allocate an HPT one order above what
> > +     * we'd normally give them (to stop a small guest claiming a huge
> > +     * chunk of resources in the HPT */
> > +    if (shift > (spapr_hpt_shift_for_ramsize(current_ram_size) + 1)) {
> > +        return H_RESOURCE;
> > +    }
> >
> 
> In your previous version, get_current_ram_size() returned ram_size + dimm
> size, but here,  pc_existing_dimms_capacity() gives only the dimm size. I
> guess you need to add ram_size to it ?

Oops, thanks for catching that.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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