Re: [PATCH V5 22/22] xen/arm: Add mapcache invalidation handling

[Oleksandr <olekstysh@gmail.com>]

On 28.01.21 11:55, Julien Grall wrote:


> Hi Oleksandr,

Hi Julien


>
> On 25/01/2021 19:08, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>> 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.
>
> AFAICT, this sentence doesn't match the code. Instead, you will 
> request the mapcache invalidation whenever the "freed" entry contains 
> some RAM page.
>
> I am fine with the approach used in the code. However, it would be 
> good that the commit message reflects the code.

ok, will update it.


>
>
>> 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.
>>
>> Taking into the account the following the do_trap_hypercall()
>> is the best place to send invalidation request:
>>   - The only way a guest can modify its P2M on Arm is via an hypercall
>>   - When sending the invalidation request, the vCPU will be blocked
>>     until all the IOREQ servers have acknowledged the invalidation
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>> CC: Julien Grall <julien.grall@arm.com>
>> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
>> [On Arm only]
>> Tested-by: Wei Chen <Wei.Chen@arm.com>
>>
>> ---
>> Please note, this is a split/cleanup/hardening of Julien's PoC:
>> "Add support for Guest IO forwarding to a device emulator"
>>
>> ***
>> Please note, this patch depends on the following which is
>> on review:
>> https://patchwork.kernel.org/patch/11803383/
>
> I think we can defer this post this series.
ok
>
>
>>
>> This patch is on par with x86 code (whether it is buggy or not).
>> If there is a need to improve/harden something, this can be done on
>> a follow-up.
>> ***
>>
>> 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()
>>
>> Changes V3 -> V4:
>>     - update patch description
>>     - re-order check in p2m_free_entry() to call 
>> domain_has_ioreq_server()
>>       only if p2m->domain == current->domain
>>     - add a comment in do_trap_hypercall()
>>
>> Changes V4 -> V5:
>>     - add Stefano's R-b
>>     - update comment in do_trap_hypercall()
>> ---
>> ---
>>   xen/arch/arm/p2m.c   | 25 +++++++++++++++++--------
>>   xen/arch/arm/traps.c | 20 +++++++++++++++++---
>>   2 files changed, 34 insertions(+), 11 deletions(-)
>>
>> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
>> index d41c4fa..26acb95d 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 +750,25 @@ 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
>
> I am not sure why you specify "non-recursively". You still want to 
> invalidate the mapcache if we replaced a table with a superpage mapping.

yes agree, sorry I don't quite remember why I decided to put emphasis on 
this word. I will just remove it.


>
>
>> +         * was replaced by an entry with a different base (valid 
>> case) or
>> +         * the shattering of a superpage was failed (error case).
>
> s/was/has/ I think.

ok, I assume this applies for the second part of the sentence.


>
>> +         * So, at worst, the spurious mapcache invalidation might be 
>> sent.
>> +         */
>> +        if ( (p2m->domain == current->domain) &&
>> +              domain_has_ioreq_server(p2m->domain) &&
>> +              p2m_is_ram(entry.p2m.type) )
>> +            p2m->domain->mapcache_invalidate = true;
>> +#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 4cdd343..64b740b 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;
>>         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 +1490,21 @@ 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
>> +    /*
>> +     * We call ioreq_signal_mapcache_invalidate from 
>> do_trap_hypercall()
>> +     * because the only way a guest can modify its P2M on Arm is via an
>> +     * hypercall.
>> +     * Note that sending the invalidation request causes the vCPU to 
>> block
>> +     * until all the IOREQ servers have acknowledged the invalidation.
>> +     */
>> +    if ( unlikely(curr->domain->mapcache_invalidate) &&
>> + test_and_clear_bool(curr->domain->mapcache_invalidate) )
>> +        ioreq_signal_mapcache_invalidate();
>> +#endif
>>   }
>>     void arch_hypercall_tasklet_result(struct vcpu *v, long res)
>>
>
> Cheers,
>
-- 
Regards,

Oleksandr Tyshchenko