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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Jan Beulich" <jbeulich@suse.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Cc: <xen-devel@lists.xenproject.org>,
	Ian Jackson <iwj@xenproject.org>,
	Anthony PERARD <anthony.perard@citrix.com>
Subject: Re: [PATCH v2] SUPPORT.md: Un-shimmed 32-bit PV guests are no longer supported
Date: Thu, 6 May 2021 15:32:25 +0100	[thread overview]
Message-ID: <5b4c9bce-9335-b5f7-0c90-e47ae264d896@citrix.com> (raw)
In-Reply-To: <acd695c1-dc04-4606-5212-5fd993e355b1@suse.com>

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,

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.


  reply	other threads:[~2021-05-06 14:32 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-06 12:47 George Dunlap
2021-05-06 13:09 ` Jan Beulich
2021-05-06 14:32   ` Andrew Cooper [this message]
2021-05-07 10:26 ` Roger Pau Monné
2021-05-07 11:05   ` Jan Beulich
2021-06-04 13:14 ` George Dunlap
2021-06-07  9:18 ` Jan Beulich

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