Re: [PATCH v6 1/9] xen/arm: introduce static shared memory

[Julien Grall <julien@xxxxxxx>]

Hi Penny,

On 29/08/2022 07:57, Penny Zheng wrote:
> > -----Original Message-----
> > From: Julien Grall <julien@xxxxxxx>
> > Sent: Friday, August 26, 2022 9:17 PM
> > To: Penny Zheng <Penny.Zheng@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Bertrand Marquis
> > <Bertrand.Marquis@xxxxxxx>; Volodymyr Babchuk
> > <Volodymyr_Babchuk@xxxxxxxx>
> > Subject: Re: [PATCH v6 1/9] xen/arm: introduce static shared memory
> >
> > Hi Penny,
> Hi Julien
>   
> > On 21/07/2022 14:21, Penny Zheng wrote:
> > > From: Penny Zheng <penny.zheng@xxxxxxx>
> > >
> > > This patch series introduces a new feature: setting up static shared
> > > memory on a dom0less system, through device tree configuration.
> > >
> > > This commit parses shared memory node at boot-time, and reserve it in
> > > bootinfo.reserved_mem to avoid other use.
> > >
> > > This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
> > > static-shm-related codes, and this option depends on static memory(
> > > CONFIG_STATIC_MEMORY). That's because that later we want to reuse a
> > > few helpers, guarded with CONFIG_STATIC_MEMORY, like
> > > acquire_staticmem_pages, etc, on static shared memory.
> > >
> > > Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
> > > ---
> > > v6 change:
> > > - when host physical address is ommited, output the error message
> > > since xen doesn't support it at the moment
> > > - add the following check: 1) The shm ID matches and the region
> > > exactly match
> > > 2) The shm ID doesn't match and the region doesn't overlap
> > > - change it to "unsigned int" to be aligned with nr_banks
> > > - check the len of the property to confirm is it big enough to contain
> > > "paddr", "size", and "gaddr"
> > > - shm_id defined before nr_shm_domain, so we could re-use the existing
> > > hole and avoid increasing the size of the structure.
> > > - change "nr_shm_domain" to "nr_shm_borrowers", to not increment if
> > > the role is owner in parsing code
> > > - make "xen,shm_id" property as arbitrary string, with a strict limit
> > > on the number of characters, MAX_SHM_ID_LENGTH
> > > ---
> > > v5 change:
> > > - no change
> > > ---
> > > v4 change:
> > > - nit fix on doc
> > > ---
> > > v3 change:
> > > - make nr_shm_domain unsigned int
> > > ---
> > > v2 change:
> > > - document refinement
> > > - remove bitmap and use the iteration to check
> > > - add a new field nr_shm_domain to keep the number of shared domain
> > > ---
> > >    docs/misc/arm/device-tree/booting.txt | 124 ++++++++++++++++++++
> > >    xen/arch/arm/Kconfig                  |   6 +
> > >    xen/arch/arm/bootfdt.c                | 157 ++++++++++++++++++++++++++
> > >    xen/arch/arm/include/asm/setup.h      |   9 ++
> > >    4 files changed, 296 insertions(+)
> > >
> > > diff --git a/docs/misc/arm/device-tree/booting.txt
> > > b/docs/misc/arm/device-tree/booting.txt
> > > index 98253414b8..8013fb98fe 100644
> > > --- a/docs/misc/arm/device-tree/booting.txt
> > > +++ b/docs/misc/arm/device-tree/booting.txt
> > > @@ -378,3 +378,127 @@ device-tree:
> > >
> > >    This will reserve a 512MB region starting at the host physical address
> > >    0x30000000 to be exclusively used by DomU1.
> > > +
> > > +Static Shared Memory
> > > +====================
> > > +
> > > +The static shared memory device tree nodes allow users to statically
> > > +set up shared memory on dom0less system, enabling domains to do
> > > +shm-based communication.
> > > +
> > > +- compatible
> > > +
> > > +    "xen,domain-shared-memory-v1"
> > > +
> > > +- xen,shm-id
> > > +
> > > +    An arbitrary string that represents the unique identifier of the shared
> > > +    memory region, with a strict limit on the number of characters(\0
> > included),
> > > +    `MAX_SHM_ID_LENGTH(16)`. e.g. "xen,shm-id = "my-shared-mem-1"".
> > > +
> > > +- xen,shared-mem
> > > +
> > > +    An array takes a physical address, which is the base address of the
> > > +    shared memory region in host physical address space, a size, and a
> > guest
> > > +    physical address, as the target address of the mapping.
> > > +    e.g. xen,shared-mem = < [host physical address] [size] [guest
> > > + address] >
> > Your implementation below is checking for overlap and also have some
> > restriction. Can they be documented in the binding?
> >
> > > +
> > > +    The number of cells for the host address (and size) is the same as the
> > > +    guest pseudo-physical address and they are inherited from the parent
> > node.
> >
> > In v5, we discussed to have the host address optional. However, the binding
> > has not been updated to reflect that. Note that I am not asking to implement,
> > but instead request that the binding can be used for such setup.
> How about:
> "
> Host physical address could be omitted by users, and let Xen decide where it 
> locates.
> "
I am fine with that.

> Do you think I shall further point out that right now, this part feature is not 
> implemented
> in codes?
I have made a couple of suggestion for the code. But I think the binding 
would look a bit odd without the host physical address. We would now have:
< [size] [guest address]>

I think it would be more natural if we had

<[guest address] [size]>

And

<[guest address] [size] [host physical address]>

> > > a/xen/arch/arm/include/asm/setup.h
> > b/xen/arch/arm/include/asm/setup.h
> > > index 2bb01ecfa8..39d4e93b8b 100644
> > > --- a/xen/arch/arm/include/asm/setup.h
> > > +++ b/xen/arch/arm/include/asm/setup.h
> > > @@ -23,10 +23,19 @@ typedef enum {
> > >    }  bootmodule_kind;
> > >
> > >
> > > +#ifdef CONFIG_STATIC_SHM
> > > +/* Indicates the maximum number of characters(\0 included) for shm_id
> > > +*/ #define MAX_SHM_ID_LENGTH 16 #endif
> > Is the #ifdef really needed?
> >
> > > +
> > >    struct membank {
> > >        paddr_t start;
> > >        paddr_t size;
> > >        bool xen_domain; /* whether the memory bank is bound to a Xen
> > > domain. */
> > > +#ifdef CONFIG_STATIC_SHM
> > > +    char shm_id[MAX_SHM_ID_LENGTH];
> > > +    unsigned int nr_shm_borrowers;
> > > +#endif
> > >    };
> > If I calculated right, the structure will grow from 24 to 40 bytes. At the
> > moment, this is protected with CONFIG_STATIC_SHM which is unsupported.
> > However, I think we will need to do something as we can't continue to grow
> > 'membank' like that.
> >
> > I don't have a quick suggestion for 4.17 (the feature freeze is in a week). Long
> > term, I think we will want to consider to move the shm ID in a separate array
> > that could be referenced here.
> >
> > The other solution would be to have the shared memory regions in a
> > separate array. They would have their own structure which would either
> > embedded "membank" or contain a pointer/index to the bank.
> Ok, so other than this fixing, others will be addressed in the next serie. And 
> this
> part fixing will be introduced in a new follow-up patch serie after 4.17 
> release.
>
> I'm in favor of introducing a new structure to contain shm-related data and let
> 'membank' contains a pointer to it, like
> ```
>   +struct shm_membank {
> +    char shm_id[MAX_SHM_ID_LENGTH];
> +    unsigned int nr_shm_borrowers;
> +}
> +
>   struct membank {
>       paddr_t start;
>       paddr_t size;
>       bool xen_domain; /* whether the memory bank is bound to a Xen domain. */
> +    struct shm_membank *shm;
>   };
> ```
> Then every time we introduce a new feature here, following this strategy, 
> 'membank' will
> at most grow 8 bytes for the reference.
Be aware that we are very early in Xen and therefore dynamically 
allocating memory is not possible. Therefore 'shm_membank' would most 
likely need to be static.
At which point, this could be an index.

> I'm open to the discussion and will let it decide what it finally will be. ;)
My original idea was to have 'shm_membank' pointing to the 'membank' 
rather than the other way around. But I don't have a strong argument 
either way.
So I would need to see the resulting code. Anyway, that's for post-4.17.

Cheers,

--
Julien Grall