Re: [PATCH v4 1/2] x86/mem_sharing: make fork_reset more configurable

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Apr 13, 2022 at 09:41:51AM -0400, Tamas K Lengyel wrote:
> Allow specify distinct parts of the fork VM to be reset. This is useful when a
> fuzzing operation involves mapping in only a handful of pages that are known
> ahead of time. Throwing these pages away just to be re-copied immediately is
> expensive, thus allowing to specify partial resets can speed things up.
> 
> Also allow resetting to be initiated from vm_event responses as an
> optiomization.
> 
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> v4: No change
> v3: Rebase on simpler approach after dropping empty_p2m feature
> v2: address review comments and add more sanity checking
> ---
>  tools/include/xenctrl.h                |  3 ++-
>  tools/libs/ctrl/xc_memshr.c            |  7 ++++++-
>  xen/arch/x86/include/asm/mem_sharing.h |  9 +++++++++
>  xen/arch/x86/mm/mem_sharing.c          | 24 +++++++++++++++++++-----
>  xen/common/vm_event.c                  | 15 +++++++++++++++
>  xen/include/public/memory.h            |  4 +++-
>  xen/include/public/vm_event.h          |  8 ++++++++
>  7 files changed, 62 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/include/xenctrl.h b/tools/include/xenctrl.h
> index 95bd5eca67..1b089a2c02 100644
> --- a/tools/include/xenctrl.h
> +++ b/tools/include/xenctrl.h
> @@ -2290,7 +2290,8 @@ int xc_memshr_fork(xc_interface *xch,
>   *
>   * With VMs that have a lot of memory this call may block for a long time.
>   */
> -int xc_memshr_fork_reset(xc_interface *xch, uint32_t forked_domain);
> +int xc_memshr_fork_reset(xc_interface *xch, uint32_t forked_domain,
> +                         bool reset_state, bool reset_memory);
>  
>  /* Debug calls: return the number of pages referencing the shared frame 
> backing
>   * the input argument. Should be one or greater.
> diff --git a/tools/libs/ctrl/xc_memshr.c b/tools/libs/ctrl/xc_memshr.c
> index a6cfd7dccf..a0d0b894e2 100644
> --- a/tools/libs/ctrl/xc_memshr.c
> +++ b/tools/libs/ctrl/xc_memshr.c
> @@ -257,12 +257,17 @@ int xc_memshr_fork(xc_interface *xch, uint32_t pdomid, 
> uint32_t domid,
>      return xc_memshr_memop(xch, domid, &mso);
>  }
>  
> -int xc_memshr_fork_reset(xc_interface *xch, uint32_t domid)
> +int xc_memshr_fork_reset(xc_interface *xch, uint32_t domid, bool reset_state,
> +                         bool reset_memory)
>  {
>      xen_mem_sharing_op_t mso;
>  
>      memset(&mso, 0, sizeof(mso));
>      mso.op = XENMEM_sharing_op_fork_reset;
> +    if ( reset_state )
> +        mso.u.fork.flags |= XENMEM_FORK_RESET_STATE;
> +    if ( reset_memory )
> +        mso.u.fork.flags |= XENMEM_FORK_RESET_MEMORY;

IMO would be clearer to init mso fields at definition.

> diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
> index 84cf52636b..d26a6699fc 100644
> --- a/xen/common/vm_event.c
> +++ b/xen/common/vm_event.c
> @@ -28,6 +28,11 @@
>  #include <asm/p2m.h>
>  #include <asm/monitor.h>
>  #include <asm/vm_event.h>
> +
> +#ifdef CONFIG_MEM_SHARING
> +#include <asm/mem_sharing.h>
> +#endif
> +
>  #include <xsm/xsm.h>
>  #include <public/hvm/params.h>
>  
> @@ -394,6 +399,16 @@ static int vm_event_resume(struct domain *d, struct 
> vm_event_domain *ved)
>              if ( rsp.reason == VM_EVENT_REASON_MEM_PAGING )
>                  p2m_mem_paging_resume(d, &rsp);
>  #endif
> +#ifdef CONFIG_MEM_SHARING
> +            if ( mem_sharing_is_fork(d) )
> +            {
> +                bool reset_state = rsp.flags & 
> VM_EVENT_FLAG_RESET_FORK_STATE;
> +                bool reset_mem = rsp.flags & VM_EVENT_FLAG_RESET_FORK_MEMORY;
> +
> +                if ( reset_state || reset_mem )
> +                    ASSERT(!mem_sharing_fork_reset(d, reset_state, 
> reset_mem));

Might be appropriate to destroy the domain in case fork reset fails?
ASSERT will only help in debug builds.

> +            }
> +#endif
>  
>              /*
>               * Check emulation flags in the arch-specific handler only, as it
> diff --git a/xen/include/public/memory.h b/xen/include/public/memory.h
> index a1a0f0233a..f8d26fb77d 100644
> --- a/xen/include/public/memory.h
> +++ b/xen/include/public/memory.h
> @@ -541,12 +541,14 @@ struct xen_mem_sharing_op {
>                  uint32_t gref;     /* IN: gref to debug         */
>              } u;
>          } debug;
> -        struct mem_sharing_op_fork {      /* OP_FORK */
> +        struct mem_sharing_op_fork {      /* OP_FORK{,_RESET} */
>              domid_t parent_domain;        /* IN: parent's domain id */
>  /* Only makes sense for short-lived forks */
>  #define XENMEM_FORK_WITH_IOMMU_ALLOWED (1u << 0)
>  /* Only makes sense for short-lived forks */
>  #define XENMEM_FORK_BLOCK_INTERRUPTS   (1u << 1)

Should you add:

/* Only for OP_FORK_RESET. */

> +#define XENMEM_FORK_RESET_STATE        (1u << 2)
> +#define XENMEM_FORK_RESET_MEMORY       (1u << 3)

Thanks, Roger.