From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: elena.ufimtseva@oracle.com, ian.campbell@citrix.com,
stefano.stabellini@eu.citrix.com, andrew.cooper3@citrix.com,
tim@xen.org, linux-kernel@vger.kernel.org,
david.vrabel@citrix.com, jbeulich@suse.com,
xen-devel@lists.xenproject.org, ian.jackson@eu.citrix.com,
roger.pau@citrix.com
Subject: Re: [PATCH v2 4/6] xen/x86/pvh: Set up descriptors for 32-bit PVH guests
Date: Tue, 21 Jul 2015 11:32:43 -0400 [thread overview]
Message-ID: <55AE661B.90909__36086.8055992343$1437492883$gmane$org@oracle.com> (raw)
In-Reply-To: <55A940CC.6090509@oracle.com>
On 07/17/2015 01:52 PM, Boris Ostrovsky wrote:
> On 07/17/2015 12:43 PM, Konrad Rzeszutek Wilk wrote:
>> On Fri, Jul 17, 2015 at 11:36:29AM -0400, Boris Ostrovsky wrote:
>>> On 07/17/2015 11:21 AM, Konrad Rzeszutek Wilk wrote:
>>>> On Thu, Jul 16, 2015 at 05:43:39PM -0400, Boris Ostrovsky wrote:
>>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>>>> ---
>>>>> Changes in v2:
>>>>> * Set segment selectors using loadsegment() instead of assembly
>>>>>
>>>>> arch/x86/xen/enlighten.c | 15 ++++++++++-----
>>>>> 1 file changed, 10 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>>>>> index f8dc398..d665b1d 100644
>>>>> --- a/arch/x86/xen/enlighten.c
>>>>> +++ b/arch/x86/xen/enlighten.c
>>>>> @@ -1362,12 +1362,12 @@ static void __init
>>>>> xen_boot_params_init_edd(void)
>>>>> static void __ref xen_setup_gdt(int cpu)
>>>>> {
>>>>> if (xen_feature(XENFEAT_auto_translated_physmap)) {
>>>>> -#ifdef CONFIG_X86_64
>>>>> - unsigned long dummy;
>>>>> + unsigned long __attribute__((unused)) dummy;
>>>>> - load_percpu_segment(cpu); /* We need to access per-cpu
>>>>> area */
>>>> You removed that - where are we going to do that? As the
>>>> 'switch_to_new_gdt' uses the per-cpu GDT table.
>>> load_percpu_segment() is part of switch_to_new_gdt(), so I thought
>>> there is
>>> no need to call it here.
>>>
>>> But you are right --- switch_to_new_gdt() starts with
>>> get_cpu_gdt_table()
>>> which accesses per-CPU area. How did this manage to work then?
>> I was surprised as well - I was expecting your patch to have blow up.
>> Unless we are doing something fancy for CPU0 and for the other CPUs we
>> already have the per-cpu segment setup during bootup (copied from BSP)?
>
>
> No, %fs is zero when we enter xen_setup_gdt() (for 32-bit).
>
> In any case, I should put load_percpu_segment() back.
No, I shouldn't.
Until the new GDT is loaded we can't load selectors since current GDT
doesn't have descriptors set up for them. And so any attempt to load
uninitialized selectors results in a fault.
This worked for 64-bit guests because there we load zero into %gs and
that is allowed (processor doesn't perform descriptor checks for the
first 4 indexes). But for 32-bit guests we load %fs with 0xd8.
And the reason the code worked before was because we are using "master"
per-cpu area and because GDT is the same for all CPUs at that point. Or
so I think.
-boris
next prev parent reply other threads:[~2015-07-21 15:33 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-16 21:43 [PATCH v2 0/6] 32-bit PVH domU support Boris Ostrovsky
2015-07-16 21:43 ` [PATCH v2 1/6] xen/x86/pvh: Save %rbx in xen_pvh_early_cpu_init() Boris Ostrovsky
2015-07-16 21:43 ` [PATCH v2 2/6] xen/x86: Remove unnecessary memset() call Boris Ostrovsky
2015-07-16 21:43 ` [PATCH v2 3/6] xen/x86/pvh: Properly set page tables for 32-bit PVH guests Boris Ostrovsky
2015-07-16 21:43 ` [PATCH v2 4/6] xen/x86/pvh: Set up descriptors " Boris Ostrovsky
2015-07-17 15:21 ` Konrad Rzeszutek Wilk
[not found] ` <20150717152113.GB18085@l.oracle.com>
2015-07-17 15:36 ` Boris Ostrovsky
[not found] ` <55A920FD.6050101@oracle.com>
2015-07-17 16:43 ` Konrad Rzeszutek Wilk
[not found] ` <20150717164331.GA19827@l.oracle.com>
2015-07-17 17:52 ` Boris Ostrovsky
[not found] ` <55A940CC.6090509@oracle.com>
2015-07-21 15:32 ` Boris Ostrovsky [this message]
2015-07-16 21:43 ` [PATCH v2 5/6] xen/x86/pvh: Add 32-bit PVH initialization code Boris Ostrovsky
2015-07-16 21:43 ` [PATCH v2 6/6] xen/x86/pvh: Allow building 32-bit PVH guests Boris Ostrovsky
[not found] ` <1437083021-24488-2-git-send-email-boris.ostrovsky@oracle.com>
2015-07-17 15:09 ` [PATCH v2 1/6] xen/x86/pvh: Save %rbx in xen_pvh_early_cpu_init() Konrad Rzeszutek Wilk
[not found] ` <1437083021-24488-6-git-send-email-boris.ostrovsky@oracle.com>
2015-07-17 15:21 ` [PATCH v2 5/6] xen/x86/pvh: Add 32-bit PVH initialization code Konrad Rzeszutek Wilk
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