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Re: [PATCH v2] SUPPORT.md: Un-shimmed 32-bit PV guests are no longer supported
- To: Jan Beulich <jbeulich@xxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 6 May 2021 15:32:25 +0100
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- Cc: <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>
- Delivery-date: Thu, 06 May 2021 14:32:43 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06/05/2021 14:09, Jan Beulich wrote:
> On 06.05.2021 14:47, George Dunlap wrote:
>> --- a/xen/arch/x86/Kconfig
>> +++ b/xen/arch/x86/Kconfig
>> @@ -55,7 +55,7 @@ config PV
>> config PV32
>> bool "Support for 32bit PV guests"
>> depends on PV
>> - default y
>> + default PV_SHIM
>> select COMPAT
>> ---help---
>> The 32bit PV ABI uses Ring1, an area of the x86 architecture which
>> @@ -67,7 +67,10 @@ config PV32
>> reduction, or performance reasons. Backwards compatibility can be
>> provided via the PV Shim mechanism.
>>
>> - If unsure, say Y.
>> + Note that outside of PV Shim, 32-bit PV guests are not security
>> + supported anymore.
>> +
>> + If unsure, use the default setting.
> Alongside this I wonder whether we should also default opt_pv32 to false
> then, unless running in shim mode.
No - that's just rude to users.
Anyone whose enabled CONFIG_PV32 may potentially want to run such guests.
Its easy to avoid issues here by not running 32bit bit guests, or not
running untrusted guests, but forcing everyone to reboot a second time
to specify pv=32 to unbreak their environment is downright unhelpful.
Perhaps tangentially, xl/libxl needs some remedial work as a followup,
because:
Executing 'xl create -p tests/example/test-pv32pae-example.cfg'
Parsing config from tests/example/test-pv32pae-example.cfg
xc: error: panic: xg_dom_boot.c:121: xc_dom_boot_mem_init: can't
allocate low memory for domain: Out of memory
libxl: error: libxl_dom.c:586:libxl__build_dom: xc_dom_boot_mem_init
failed: Operation not supported
libxl: error: libxl_create.c:1572:domcreate_rebuild_done: Domain
3:cannot (re-)build domain: -3
libxl: error: libxl_domain.c:1182:libxl__destroy_domid: Domain
3:Non-existant domain
libxl: error: libxl_domain.c:1136:domain_destroy_callback: Domain
3:Unable to destroy guest
libxl: error: libxl_domain.c:1063:domain_destroy_cb: Domain
3:Destruction of domain failed
is what the user gets back when Xen has correctly reported that it isn't
pv32-capable, and rejects the switch_compat() hypercall.
~Andrew
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