[Xen-devel] Is: ARM maintainers advice ..Was:Re: [PATCH v5 11/28] xsplice: Implement support for applying/reverting/replacing patches.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Sat, Apr 09, 2016 at 10:36:02PM -0400, Konrad Rzeszutek Wilk wrote:
> On Thu, Apr 07, 2016 at 09:43:38AM -0600, Jan Beulich wrote:
> > >>> On 07.04.16 at 05:09, <konrad.wilk@xxxxxxxxxx> wrote:
> > >> > +    uint8_t *old_ptr;
> > >> > +
> > >> > +    BUILD_BUG_ON(PATCH_INSN_SIZE > sizeof(func->undo));
> > >> > +    BUILD_BUG_ON(PATCH_INSN_SIZE != (1 + sizeof val));
> > >> > +
> > >> > +    old_ptr = (uint8_t *)func->old_addr;
> > >> 
> > >> (Considering this cast, the "old_addr" member should be
> > >> unsigned long (or void *), not uint64_t: The latest on ARM32
> > >> such would otherwise cause problems.)
> > > 
> > > I has to be uint8_t to make the single byte modifications. Keep
> > > also in mind that this file is only for x86.
> > 
> > old_addr can't reasonably be uint8_t, so I can only assume you're
> > mixing up things here. (And yes, I do realize this is x86 code, but
> > my reference to ARM32 was only mean to say that there you'll
> > need to do something similar, and casting uint64_t to whatever
> > kind of pointer type is not going to work without compiler warning.)
> 
> Way back .. when we spoke about the .xsplice.funcs structure
> you recommended to make the types be either uintXX specific
> or Elf types. I choose Elf types but then we realized that
> ARM32 hypervisor would be involved which of course would have
> a different size of the structure. So I went with uintXXX
> to save a bit of headache (specifically those BUILD_BUG_ON
> checks).
> 
> I can't see making the old_addr be unsigned long or void *,
> which means going back to Elf types. And for ARM32 I will
> have to deal with a different structure size. 

CC-ing Stefano and Julien here to advise. I ended up 
exposing the ABI part in sysctl.h (and design document as):


#define XSPLICE_PAYLOAD_VERSION 1
/*
 * .xsplice.funcs structure layout defined in the `Payload format`
 * section in the xSplice design document.
 *
 * The size should be exactly 64 bytes.
 */
struct xsplice_patch_func {
    const char *name;       /* Name of function to be patched. */
    uint64_t new_addr;
    uint64_t old_addr;      /* Can be zero and name will be looked up. */
    uint32_t new_size;
    uint32_t old_size;
    uint8_t version;        /* MUST be XSPLICE_PAYLOAD_VERSION. */
    uint8_t pad[31];        /* MUST be zero filled. */
};
typedef struct xsplice_patch_func xsplice_patch_func_t;

Which looks nice.

When the ELF file is loaded we load it as this structure:

[x86]
#ifndef CONFIG_ARM
struct xsplice_patch_func_internal {
    const char *name;
    void *new_addr;
    void *old_addr;
    uint32_t new_size;
    uint32_t old_size;
    uint8_t version;
    union {
        uint8_t undo[8];
        uint8_t pad[31];
    } u;
};
#else
[ARM]
struct xsplice_patch_func_internal {
    const char *name;
    uint32_t _pad0;
    void *new_addr;
    uint32_t _pad1;
    void *old_addr;
    uint32_t _pad2;
    uint32_t new_size;
    uint32_t old_size;
    uint8_t version;
    union {
        uint8_t pad[31];
    } u;
};
#endif

That allows the size and offsets to be the same. I checked under ARM32
builds:


struct xsplice_patch_func_internal {
    const char  *              name;                 /*     0     4 */
    uint32_t                   _pad0;                /*     4     4 */
    void *                     new_addr;             /*     8     4 */
    uint32_t                   _pad1;                /*    12     4 */
    void *                     old_addr;             /*    16     4 */
    uint32_t                   _pad2;                /*    20     4 */
    uint32_t                   new_size;             /*    24     4 */
    uint32_t                   old_size;             /*    28     4 */
    uint8_t                    version;              /*    32     1 */
    union {
        uint8_t            pad[31];              /*          31 */
    } u;                                             /*    33    31 */
    /* --- cacheline 1 boundary (64 bytes) --- */

    /* size: 64, cachelines: 1, members: 10 */
};

So it all looks correct. (and I can cast the old_addr to uint8_t).
Naturally we can expand the pad[] to hold whatever is needed
when implementing xSplice under ARM

However I would appreciate advise from ARM folks if I made some
wrong assumptions..


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