Re: [Xen-devel] [RFC][PATCH 10/13] tools: extend XENMEM_set_memory_map

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

On 2015/5/15 15:32, Jan Beulich wrote:
> > > > On 15.05.15 at 09:09, <tiejun.chen@xxxxxxxxx> wrote:
> > On 2015/5/15 14:16, Jan Beulich wrote:
> > > > > > On 15.05.15 at 04:57, <tiejun.chen@xxxxxxxxx> wrote:
> > > > On 2015/4/20 21:51, Jan Beulich wrote:
> > > > > > > > On 10.04.15 at 11:22, <tiejun.chen@xxxxxxxxx> wrote:
> > > > > > --- a/tools/libxl/libxl_dom.c
> > > > > > +++ b/tools/libxl/libxl_dom.c
> > > > > > @@ -787,6 +787,70 @@ out:
> > > > > >         return rc;
> > > > > >     }
> > > > > >
> > > > > > +static int libxl__domain_construct_memmap(libxl_ctx *ctx,
> > > > > > +                                          libxl_domain_config *d_config,
> > > > > > +                                          uint32_t domid,
> > > > > > +                                          struct xc_hvm_build_args *args,
> > > > > > +                                          int num_pcidevs,
> > > > > > +                                          libxl_device_pci *pcidevs)
> > > > > > +{
> > > > > > +    unsigned int nr = 0, i;
> > > > > > +    /* We always own at least one lowmem entry. */
> > > > > > +    unsigned int e820_entries = 1;
> > > > > > +    uint64_t highmem_end = 0, highmem_size = args->mem_size - 
> > > > > > args->lowmem_size;
> > > > > > +    struct e820entry *e820 = NULL;
> > > > > > +
> > > > > > +    /* Add all rdm entries. */
> > > > > > +    e820_entries += d_config->num_rdms;
> > > > > > +
> > > > > > +    /* If we should have a highmem range. */
> > > > > > +    if (highmem_size)
> > > > > > +    {
> > > > > > +        highmem_end = (1ull<<32) + highmem_size;
> > > > > > +        e820_entries++;
> > > > > > +    }
> > > > > > +
> > > > > > +    e820 = malloc(sizeof(struct e820entry) * e820_entries);
> > > > > > +    if (!e820) {
> > > > > > +        return -1;
> > > > > > +    }
> > > > > > +
> > > > > > +    /* Low memory */
> > > > > > +    e820[nr].addr = 0x100000;
> > > > > > +    e820[nr].size = args->lowmem_size - 0x100000;
> > > > > > +    e820[nr].type = E820_RAM;
> > > > > If you really mean it to be this lax (not covering the low 1Mb), then
> > > > > you need to explain why in a comment (and the consuming side
> > > > > should also have a similar explanation then).
> > > > Okay, here may need this,
> > > >
> > > > /*
> > > >
> > > >     * Low RAM starts at least from 1M to make sure all standard regions
> > > >
> > > >     * of the PC memory map, like BIOS, VGA memory-mapped I/O and vgabios,
> > > >
> > > >     * have enough space.
> > > >     */
> > > > #define GUEST_LOW_MEM_START_DEFAULT 0x100000
> > > But this only states a generic fact, but doesn't explain why you can
> > > lump together all the different things below 1Mb into a single E820
> > > entry.
> > I'm not sure if I'm misleading you. All different things below 1M is not
> > in a single entry. Here we just construct these mappings:
> >
> > #1. [1M, lowmem_end]
> > #2. [RDM]
> > #3. [4G, highmem_end]
> >
> > Those stuffs below 1M are still constructed with multiple e820 entries
> > by hvmloader. At this point I don't change anything.
> Then _that_ is what the comment needs to say.
>
> > > > > > +    nr++;
> > > > > > +
> > > > > > +    /* RDM mapping */
> > > > > > +    for (i = 0; i < d_config->num_rdms; i++) {
> > > > > > +        /*
> > > > > > +         * We should drop this kind of rdm entry.
> > > > > > +         */
> > > > > > +        if (d_config->rdms[i].flag == LIBXL_RDM_RESERVE_FLAG_INVALID)
> > > > > > +            continue;
> > > > > > +
> > > > > > +        e820[nr].addr = d_config->rdms[i].start;
> > > > > > +        e820[nr].size = d_config->rdms[i].size;
> > > > > > +        e820[nr].type = E820_RESERVED;
> > > > > > +        nr++;
> > > > > > +    }
> > > > > Is this guaranteed not to produce overlapping entries?
> > > > Right, I would add this at the beginning,
> > > >
> > > >        if (e820_entries >= E820MAX) {
> > > >            LOG(ERROR, "Ooops! Too many entries in the memory map!\n");
> > > >            return -1;
> > > >        }
> > > That would be a protection against too many entries, but not against
> > > overlapping ones.
> > Are you saying these kinds of mapping?
> >
> > #1. [1M, lowmem_end]
> > #2. [RDM]
> > #3. [4G, highmem_end]
> >
> > Before we call this function we already finish handling RDM with our
> > policy. This means there's no any overlapping here.
> That would be fine then. Note what I had asked: "Is this guaranteed
> not to produce overlapping entries?" I.e. if it is guaranteed (which
> afaict isn't obvious from the code itself), then please again say why
> in a brief comment.
Sorry for this unclear reply previously.

I would summary current reply as a comment.

Thanks
Tiejun

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