Re: [PATCH] xen/common: Do not allocate magic pages 1:1 for direct mapped domains

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Henry,

On 26/02/2024 02:19, Henry Wang wrote:
> An error message can seen from the init-dom0less application on
> direct-mapped 1:1 domains:
> ```
> Allocating magic pages
> memory.c:238:d0v0 mfn 0x39000 doesn't belong to d1
> Error on alloc magic pages
> ```
> 
> This is because populate_physmap() automatically assumes gfn == mfn
> for direct mapped domains. This cannot be true for the magic pages
> that are allocated later for Dom0less DomUs from the init-dom0less
> helper application executed in Dom0.
> 
> Force populate_physmap to take the "normal" memory allocation route for
> the magic pages even for 1:1 Dom0less DomUs. This should work as long
> as the 1:1 Dom0less DomU doesn't have anything else mapped at the same
> guest address as the magic pages:
> - gfn 0x39000 address 0x39000000
> - gfn 0x39001 address 0x39001000
> - gfn 0x39002 address 0x39002000
> - gfn 0x39003 address 0x39003000
> Create helper is_magic_gpfn() for Arm to assist this and stub helpers
> for non-Arm architectures to avoid #ifdef. Move the definition of the
> magic pages on Arm to a more common place.
> 
> Note that the init-dom0less application of the diffenent Xen version
> may allocate all or part of four magic pages for each DomU.
> 
> Reported-by: Alec Kwapis <alec.kwapis@xxxxxxxxxxxxx>
> Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
NIT: Generally, the first SOB is the same as author of the patch.

[...]
> diff --git a/xen/common/memory.c b/xen/common/memory.c
> index b3b05c2ec0..ab4bad79e2 100644
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c
> @@ -219,7 +219,7 @@ static void populate_physmap(struct memop_args *a)
>          }
>          else
>          {
> -            if ( is_domain_direct_mapped(d) )
> +            if ( is_domain_direct_mapped(d) && !is_magic_gpfn(gpfn) )
I struggle to understand the goal of this patch and the proposed solution.
The problem with magic pages applies to static mem domUs in general.
A direct mapped domU is a static mem domU whose memory is 1:1 mapped.
Let's say we try to map a magic page for a direct mapped domU. That check will 
be false
and the execution will move to the next one i.e. is_domain_using_staticmem(d).
This check will be true and acquire_reserved_page() will fail instead (similar 
to the
static mem (no direct map) scenario). The only thing that has changed is the 
message.

~Michal