Re: [PATCH for-5.2] Revert series "spapr/xive: Allocate vCPU IPIs from the vCPU contexts"

[Cédric Le Goater]

On 11/16/20 4:34 PM, Greg Kurz wrote:
> This series was largely built on the assumption that IPI numbers are
> numerically equal to vCPU ids. Even if this happens to be the case
> in practice with the default machine settings, this ceases to be true
> if VSMT is set to a different value than the number of vCPUs per core.
> This causes bogus IPI numbers to be created in KVM from a guest stand
> point. This leads to unknow results in the guest, including crashes
> or missing vCPUs (see BugLink) and even non-fatal oopses in current
> KVM that lacks a check before accessing misconfigured HW (see [1]).
> 
> A tentative patch was sent (see [2]) but it seems too complex to be
> merged in an RC. Since the original changes are essentially an
> optimization, it seems safer to revert them for now. The damage is
> done by commit acbdb9956fe9 ("spapr/xive: Allocate IPIs independently
> from the other sources") but the previous patches in the series are
> really preparatory patches. So this reverts the whole series:
> 
> eab0a2d06e97 ("spapr/xive: Allocate vCPU IPIs from the vCPU contexts")
> acbdb9956fe9 ("spapr/xive: Allocate IPIs independently from the other 
> sources")

These are introducing the optimisation to allocate the vCPU IPI from the 
running task, and, at the same time, the issue for guests using vSMT.

> fa94447a2cd6 ("spapr/xive: Use kvmppc_xive_source_reset() in post_load")
> 235d3b116213 ("spapr/xive: Modify kvm_cpu_is_enabled() interface")

IMO, these two last patches are fine. 

C. 

 
> [1] https://marc.info/?l=kvm-ppc&m=160458409722959&w=4
> [2] https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg03626.html
> 
> Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
> Fixes: acbdb9956fe9 ("spapr/xive: Allocate IPIs independently from the other 
> sources")
> BugLink: https://bugs.launchpad.net/qemu/+bug/1900241
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
> 
> Peter,
> 
> I'm Cc'ing you because we really want to fix this regression in 5.2.
> Reverting the culprit optimization seems a lot safer than the changes
> proposed in my other patch. David is on PTO right now and I'm not sure
> if he can post a PR anytime soon. Just in case: would it be acceptable
> to you if I send a PR after it got some positive feedback from the
> people on the Cc: list ? The better the sooner or do we wait for David
> to get a chance to step in ?
> ---
>  hw/intc/spapr_xive_kvm.c |  102 
> ++++++++--------------------------------------
>  1 file changed, 18 insertions(+), 84 deletions(-)
> 
> diff --git a/hw/intc/spapr_xive_kvm.c b/hw/intc/spapr_xive_kvm.c
> index 66bf4c06fe55..e8667ce5f621 100644
> --- a/hw/intc/spapr_xive_kvm.c
> +++ b/hw/intc/spapr_xive_kvm.c
> @@ -36,9 +36,10 @@ typedef struct KVMEnabledCPU {
>  static QLIST_HEAD(, KVMEnabledCPU)
>      kvm_enabled_cpus = QLIST_HEAD_INITIALIZER(&kvm_enabled_cpus);
>  
> -static bool kvm_cpu_is_enabled(unsigned long vcpu_id)
> +static bool kvm_cpu_is_enabled(CPUState *cs)
>  {
>      KVMEnabledCPU *enabled_cpu;
> +    unsigned long vcpu_id = kvm_arch_vcpu_id(cs);
>  
>      QLIST_FOREACH(enabled_cpu, &kvm_enabled_cpus, node) {
>          if (enabled_cpu->vcpu_id == vcpu_id) {
> @@ -146,45 +147,6 @@ int kvmppc_xive_cpu_synchronize_state(XiveTCTX *tctx, 
> Error **errp)
>      return s.ret;
>  }
>  
> -/*
> - * Allocate the vCPU IPIs from the vCPU context. This will allocate
> - * the XIVE IPI interrupt on the chip on which the vCPU is running.
> - * This gives a better distribution of IPIs when the guest has a lot
> - * of vCPUs. When the vCPUs are pinned, this will make the IPI local
> - * to the chip of the vCPU. It will reduce rerouting between interrupt
> - * controllers and gives better performance.
> - */
> -typedef struct {
> -    SpaprXive *xive;
> -    Error *err;
> -    int rc;
> -} XiveInitIPI;
> -
> -static void kvmppc_xive_reset_ipi_on_cpu(CPUState *cs, run_on_cpu_data arg)
> -{
> -    unsigned long ipi = kvm_arch_vcpu_id(cs);
> -    XiveInitIPI *s = arg.host_ptr;
> -    uint64_t state = 0;
> -
> -    s->rc = kvm_device_access(s->xive->fd, KVM_DEV_XIVE_GRP_SOURCE, ipi,
> -                              &state, true, &s->err);
> -}
> -
> -static int kvmppc_xive_reset_ipi(SpaprXive *xive, CPUState *cs, Error **errp)
> -{
> -    XiveInitIPI s = {
> -        .xive = xive,
> -        .err  = NULL,
> -        .rc   = 0,
> -    };
> -
> -    run_on_cpu(cs, kvmppc_xive_reset_ipi_on_cpu, RUN_ON_CPU_HOST_PTR(&s));
> -    if (s.err) {
> -        error_propagate(errp, s.err);
> -    }
> -    return s.rc;
> -}
> -
>  int kvmppc_xive_cpu_connect(XiveTCTX *tctx, Error **errp)
>  {
>      ERRP_GUARD();
> @@ -195,7 +157,7 @@ int kvmppc_xive_cpu_connect(XiveTCTX *tctx, Error **errp)
>      assert(xive->fd != -1);
>  
>      /* Check if CPU was hot unplugged and replugged. */
> -    if (kvm_cpu_is_enabled(kvm_arch_vcpu_id(tctx->cs))) {
> +    if (kvm_cpu_is_enabled(tctx->cs)) {
>          return 0;
>      }
>  
> @@ -214,12 +176,6 @@ int kvmppc_xive_cpu_connect(XiveTCTX *tctx, Error **errp)
>          return ret;
>      }
>  
> -    /* Create/reset the vCPU IPI */
> -    ret = kvmppc_xive_reset_ipi(xive, tctx->cs, errp);
> -    if (ret < 0) {
> -        return ret;
> -    }
> -
>      kvm_cpu_enable(tctx->cs);
>      return 0;
>  }
> @@ -279,12 +235,6 @@ int kvmppc_xive_source_reset_one(XiveSource *xsrc, int 
> srcno, Error **errp)
>  
>      assert(xive->fd != -1);
>  
> -    /*
> -     * The vCPU IPIs are now allocated in kvmppc_xive_cpu_connect()
> -     * and not with all sources in kvmppc_xive_source_reset()
> -     */
> -    assert(srcno >= SPAPR_XIRQ_BASE);
> -
>      if (xive_source_irq_is_lsi(xsrc, srcno)) {
>          state |= KVM_XIVE_LEVEL_SENSITIVE;
>          if (xsrc->status[srcno] & XIVE_STATUS_ASSERTED) {
> @@ -296,28 +246,12 @@ int kvmppc_xive_source_reset_one(XiveSource *xsrc, int 
> srcno, Error **errp)
>                               true, errp);
>  }
>  
> -/*
> - * To be valid, a source must have been claimed by the machine (valid
> - * entry in the EAS table) and if it is a vCPU IPI, the vCPU should
> - * have been enabled, which means the IPI has been allocated in
> - * kvmppc_xive_cpu_connect().
> - */
> -static bool xive_source_is_valid(SpaprXive *xive, int i)
> -{
> -    return xive_eas_is_valid(&xive->eat[i]) &&
> -        (i >= SPAPR_XIRQ_BASE || kvm_cpu_is_enabled(i));
> -}
> -
>  static int kvmppc_xive_source_reset(XiveSource *xsrc, Error **errp)
>  {
>      SpaprXive *xive = SPAPR_XIVE(xsrc->xive);
>      int i;
>  
> -    /*
> -     * Skip the vCPU IPIs. These are created/reset when the vCPUs are
> -     * connected in kvmppc_xive_cpu_connect()
> -     */
> -    for (i = SPAPR_XIRQ_BASE; i < xsrc->nr_irqs; i++) {
> +    for (i = 0; i < xsrc->nr_irqs; i++) {
>          int ret;
>  
>          if (!xive_eas_is_valid(&xive->eat[i])) {
> @@ -399,7 +333,7 @@ static void kvmppc_xive_source_get_state(XiveSource *xsrc)
>      for (i = 0; i < xsrc->nr_irqs; i++) {
>          uint8_t pq;
>  
> -        if (!xive_source_is_valid(xive, i)) {
> +        if (!xive_eas_is_valid(&xive->eat[i])) {
>              continue;
>          }
>  
> @@ -582,7 +516,7 @@ static void kvmppc_xive_change_state_handler(void 
> *opaque, int running,
>              uint8_t pq;
>              uint8_t old_pq;
>  
> -            if (!xive_source_is_valid(xive, i)) {
> +            if (!xive_eas_is_valid(&xive->eat[i])) {
>                  continue;
>              }
>  
> @@ -610,7 +544,7 @@ static void kvmppc_xive_change_state_handler(void 
> *opaque, int running,
>      for (i = 0; i < xsrc->nr_irqs; i++) {
>          uint8_t pq;
>  
> -        if (!xive_source_is_valid(xive, i)) {
> +        if (!xive_eas_is_valid(&xive->eat[i])) {
>              continue;
>          }
>  
> @@ -713,22 +647,22 @@ int kvmppc_xive_post_load(SpaprXive *xive, int 
> version_id)
>          }
>      }
>  
> -    /*
> -     * We can only restore the source config if the source has been
> -     * previously set in KVM. Since we don't do that at reset time
> -     * when restoring a VM, let's do it now.
> -     */
> -    ret = kvmppc_xive_source_reset(&xive->source, &local_err);
> -    if (ret < 0) {
> -        goto fail;
> -    }
> -
>      /* Restore the EAT */
>      for (i = 0; i < xive->nr_irqs; i++) {
> -        if (!xive_source_is_valid(xive, i)) {
> +        if (!xive_eas_is_valid(&xive->eat[i])) {
>              continue;
>          }
>  
> +        /*
> +         * We can only restore the source config if the source has been
> +         * previously set in KVM. Since we don't do that for all interrupts
> +         * at reset time anymore, let's do it now.
> +         */
> +        ret = kvmppc_xive_source_reset_one(&xive->source, i, &local_err);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +
>          ret = kvmppc_xive_set_source_config(xive, i, &xive->eat[i], 
> &local_err);
>          if (ret < 0) {
>              goto fail;
> 
>