Re: [Xen-devel] [PATCH v4 4/4] libxc: Pass e820 map to HVM/PVH guests via hvm_start_info

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Mar 21, 2018 at 09:37:09AM -0400, Boris Ostrovsky wrote:
> On 03/21/2018 06:07 AM, Roger Pau Monné wrote:
> > On Tue, Mar 20, 2018 at 09:50:52AM -0700, Maran Wilson wrote:
> >> From: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> >>
> >> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> >> Signed-off-by: Maran Wilson <maran.wilson@xxxxxxxxxx>
> >> ---
> >>  tools/libxc/xc_dom_x86.c | 29 ++++++++++++++++++++++++++++-
> >>  1 file changed, 28 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/tools/libxc/xc_dom_x86.c b/tools/libxc/xc_dom_x86.c
> >> index 0b65dab..b2d8403 100644
> >> --- a/tools/libxc/xc_dom_x86.c
> >> +++ b/tools/libxc/xc_dom_x86.c
> >> @@ -35,6 +35,8 @@
> >>  #include <xen/arch-x86/hvm/start_info.h>
> >>  #include <xen/io/protocols.h>
> >>  
> >> +#include <xen-tools/libs.h>
> >> +
> >>  #include "xg_private.h"
> >>  #include "xc_dom.h"
> >>  #include "xenctrl.h"
> >> @@ -640,6 +642,8 @@ static int alloc_magic_pages_hvm(struct xc_dom_image 
> >> *dom)
> >>              dom->cmdline_size = ROUNDUP(strlen(dom->cmdline) + 1, 8);
> >>              start_info_size += dom->cmdline_size;
> >>          }
> >> +
> >> +        start_info_size += dom->e820_entries * sizeof(*(dom->e820));
> > This is not correct because sizeof(struct e820entry) != sizeof(struct
> > hvm_modlist_entry) AFAICT. This should instead be sizeof(struct
> > hvm_modlist_entry).
> 
> 
> The area for modlist is calculated above:
> 
> start_info_size +=
>         sizeof(struct hvm_modlist_entry) * HVMLOADER_MODULE_MAX_COUNT;
> 
> (What I should do though is move this from under 'if (
> !dom->device_model )', now that we are providing this data to both HVM
> and PVH guests.

Sorry, I meant sizeof(struct hvm_memmap_table_entry) != sizeof(e820entry), so
the above should be:

start_info_size += dom->e820_entries * sizeof(struct hvm_memmap_table_entry);

> >
> >>      }
> >>      else
> >>      {
> >> @@ -1666,8 +1670,9 @@ static int bootlate_hvm(struct xc_dom_image *dom)
> >>      uint32_t domid = dom->guest_domid;
> >>      xc_interface *xch = dom->xch;
> >>      struct hvm_start_info *start_info;
> >> -    size_t start_info_size;
> >> +    size_t start_info_size, modsize;
> >>      struct hvm_modlist_entry *modlist;
> >> +    struct hvm_memmap_table_entry *memmap;
> >>      unsigned int i;
> >>  
> >>      start_info_size = sizeof(*start_info) + dom->cmdline_size;
> >> @@ -1731,7 +1736,29 @@ static int bootlate_hvm(struct xc_dom_image *dom)
> >>                              ((uintptr_t)modlist - (uintptr_t)start_info);
> >>      }
> >>  
> >> +    /*
> >> +     * Check a couple of XEN_HVM_MEMMAP_TYPEs to verify consistency with
> >> +     * their corresponding e820 numerical values.
> >> +     */
> >> +    BUILD_BUG_ON(XEN_HVM_MEMMAP_TYPE_RAM != E820_RAM);
> >> +    BUILD_BUG_ON(XEN_HVM_MEMMAP_TYPE_ACPI != E820_ACPI);
> >> +
> >> +    modsize = HVMLOADER_MODULE_MAX_COUNT *
> >> +        (sizeof(*modlist) + HVMLOADER_MODULE_CMDLINE_SIZE);
> > Hm, I'm not sure this is fully correct, but I think there are previous
> > issues in this area.
> >
> > The mapped area (start_info) is of size sizeof(*start_info) +
> > dom->cmdline_size + sizeof(struct hvm_modlist_entry) *
> > dom->num_modules. Yet here you seem to assume num_modules ==
> > HVMLOADER_MODULE_MAX_COUNT?
> 
> Yes, see my response above. We've already allocated the segment to
> accommodate HVMLOADER_MODULE_MAX_COUNT entries. Which may indeed be an
> overkill.

I'm sorry, but I don't think I follow. There's only a single
xc_map_foreign_range call that maps start_info_size space:

start_info_size = sizeof(*start_info) + dom->cmdline_size;
start_info_size += sizeof(struct hvm_modlist_entry) * dom->num_modules;

So for start_info_size bootlate_hvm takes into account the exact
number of modules used.

Yet modsize seems to assume dom->num_modules ==
HVMLOADER_MODULE_MAX_COUNT?

I think those are all previous issues in this code, TBH.

> >
> > Also the initial space calculation doesn't seem to take
> > HVMLOADER_MODULE_CMDLINE_SIZE into account at all.
> 
> 
> modlist's offset is calculated with commandline's size in mind:
> 
>     modlist = (void*)(start_info + 1) + dom->cmdline_size;

Yes, that's fine AFAICT.

> >
> > And cmdline_paddr seems to be set to point to garbage if cmdline is not
> > set.
> 
> Isn't the hvm_start_info set to zero when allocated?

Yes, but the following code in add_module_to_list seems wrong to me:

if ( cmdline )
{
    assert(strnlen(cmdline, HVMLOADER_MODULE_CMDLINE_SIZE)
           < HVMLOADER_MODULE_CMDLINE_SIZE);
    strncpy(modules_cmdline_start + HVMLOADER_MODULE_CMDLINE_SIZE * index,
            cmdline, HVMLOADER_MODULE_CMDLINE_SIZE);
}

modlist[index].cmdline_paddr =
    modules_cmdline_paddr + HVMLOADER_MODULE_CMDLINE_SIZE * index;

AFAICT it will set cmdline_paddr to point to garbage if cmdline is not
set.

Anyway, this is not introduced by your patch. I will send a couple of
patches to try to fix the already existing issues in this area.

Thanks, Roger.

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