Re: [PATCH V3 21/23] xen/arm: Add mapcache invalidation handling

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 30 Nov 2020, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> 
> We need to send mapcache invalidation request to qemu/demu everytime
> the page gets removed from a guest.
> 
> At the moment, the Arm code doesn't explicitely remove the existing
> mapping before inserting the new mapping. Instead, this is done
> implicitely by __p2m_set_entry().
> 
> So we need to recognize a case when old entry is a RAM page *and*
> the new MFN is different in order to set the corresponding flag.
> The most suitable place to do this is p2m_free_entry(), there
> we can find the correct leaf type. The invalidation request
> will be sent in do_trap_hypercall() later on.

Why is it sent in do_trap_hypercall() ?


> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> 
> ---
> Please note, this is a split/cleanup/hardening of Julien's PoC:
> "Add support for Guest IO forwarding to a device emulator"
> 
> Changes V1 -> V2:
>    - new patch, some changes were derived from (+ new explanation):
>      xen/ioreq: Make x86's invalidate qemu mapcache handling common
>    - put setting of the flag into __p2m_set_entry()
>    - clarify the conditions when the flag should be set
>    - use domain_has_ioreq_server()
>    - update do_trap_hypercall() by adding local variable
> 
> Changes V2 -> V3:
>    - update patch description
>    - move check to p2m_free_entry()
>    - add a comment
>    - use "curr" instead of "v" in do_trap_hypercall()
> ---
> ---
>  xen/arch/arm/p2m.c   | 24 ++++++++++++++++--------
>  xen/arch/arm/traps.c | 13 ++++++++++---
>  2 files changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 5b8d494..9674f6f 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1,6 +1,7 @@
>  #include <xen/cpu.h>
>  #include <xen/domain_page.h>
>  #include <xen/iocap.h>
> +#include <xen/ioreq.h>
>  #include <xen/lib.h>
>  #include <xen/sched.h>
>  #include <xen/softirq.h>
> @@ -749,17 +750,24 @@ static void p2m_free_entry(struct p2m_domain *p2m,
>      if ( !p2m_is_valid(entry) )
>          return;
>  
> -    /* Nothing to do but updating the stats if the entry is a super-page. */
> -    if ( p2m_is_superpage(entry, level) )
> +    if ( p2m_is_superpage(entry, level) || (level == 3) )
>      {
> -        p2m->stats.mappings[level]--;
> -        return;
> -    }
> +#ifdef CONFIG_IOREQ_SERVER
> +        /*
> +         * If this gets called (non-recursively) then either the entry
> +         * was replaced by an entry with a different base (valid case) or
> +         * the shattering of a superpage was failed (error case).
> +         * So, at worst, the spurious mapcache invalidation might be sent.
> +         */
> +        if ( domain_has_ioreq_server(p2m->domain) &&
> +             (p2m->domain == current->domain) && p2m_is_ram(entry.p2m.type) )
> +            p2m->domain->mapcache_invalidate = true;

Why the (p2m->domain == current->domain) check? Shouldn't we set
mapcache_invalidate to true anyway? What happens if p2m->domain !=
current->domain? We wouldn't want the domain to lose the
mapcache_invalidate notification.


> +#endif
>  
> -    if ( level == 3 )
> -    {
>          p2m->stats.mappings[level]--;
> -        p2m_put_l3_page(entry);
> +        /* Nothing to do if the entry is a super-page. */
> +        if ( level == 3 )
> +            p2m_put_l3_page(entry);
>          return;
>      }
>  
> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index b6077d2..151c626 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -1443,6 +1443,7 @@ static void do_trap_hypercall(struct cpu_user_regs 
> *regs, register_t *nr,
>                                const union hsr hsr)
>  {
>      arm_hypercall_fn_t call = NULL;
> +    struct vcpu *curr = current;

Is this just to save 3 characters?


>      BUILD_BUG_ON(NR_hypercalls < ARRAY_SIZE(arm_hypercall_table) );
>  
> @@ -1459,7 +1460,7 @@ static void do_trap_hypercall(struct cpu_user_regs 
> *regs, register_t *nr,
>          return;
>      }
>  
> -    current->hcall_preempted = false;
> +    curr->hcall_preempted = false;
>  
>      perfc_incra(hypercalls, *nr);
>      call = arm_hypercall_table[*nr].fn;
> @@ -1472,7 +1473,7 @@ static void do_trap_hypercall(struct cpu_user_regs 
> *regs, register_t *nr,
>      HYPERCALL_RESULT_REG(regs) = call(HYPERCALL_ARGS(regs));
>  
>  #ifndef NDEBUG
> -    if ( !current->hcall_preempted )
> +    if ( !curr->hcall_preempted )
>      {
>          /* Deliberately corrupt parameter regs used by this hypercall. */
>          switch ( arm_hypercall_table[*nr].nr_args ) {
> @@ -1489,8 +1490,14 @@ static void do_trap_hypercall(struct cpu_user_regs 
> *regs, register_t *nr,
>  #endif
>  
>      /* Ensure the hypercall trap instruction is re-executed. */
> -    if ( current->hcall_preempted )
> +    if ( curr->hcall_preempted )
>          regs->pc -= 4;  /* re-execute 'hvc #XEN_HYPERCALL_TAG' */
> +
> +#ifdef CONFIG_IOREQ_SERVER
> +    if ( unlikely(curr->domain->mapcache_invalidate) &&
> +         test_and_clear_bool(curr->domain->mapcache_invalidate) )
> +        ioreq_signal_mapcache_invalidate();

Why not just:

if ( unlikely(test_and_clear_bool(curr->domain->mapcache_invalidate)) )
    ioreq_signal_mapcache_invalidate();