From: Bandan Das <bsd@redhat.com>
To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: kvm@vger.kernel.org
Subject: Re: APIC_ID in apic_reg_write()
Date: Wed, 29 Apr 2015 14:54:50 -0400 [thread overview]
Message-ID: <jpgiocen4sl.fsf@redhat.com> (raw)
In-Reply-To: <55412433.7080805@siemens.com> (Jan Kiszka's message of "Wed, 29 Apr 2015 20:34:27 +0200")
Jan Kiszka <jan.kiszka@siemens.com> writes:
> Am 2015-04-29 um 18:47 schrieb Bandan Das:
>>
>> Why do we allow writes to APIC_ID ? On all _newer_ processors, it's
>> read only. The spec doesn't explicitly mention it though, or atleast
>> I couldn't find it. Does userspace have a reason to modify it ?
>
> The APIC ID is read-only for x2APIC. It remains R/W for xAPIC.
Are you sure ? In 10.4 of the SDM, there is Note that says
"In processors based on Intel microarchitecture code name Nehalem the Local APIC ID
Register is no longer Read/Write; it is Read Only."
And I can verify on a SandyBridge and Haswell system that it's RO there too.
In fact, that was one of the reasons I had submitted a patch to remove
verify_local_APIC() from x86/kernel/apic.c (4399c03c678) If I am wrong we need to
revert atleast the associated commit message :)
> Jan
next prev parent reply other threads:[~2015-04-29 18:54 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-29 16:47 APIC_ID in apic_reg_write() Bandan Das
2015-04-29 18:34 ` Jan Kiszka
2015-04-29 18:54 ` Bandan Das [this message]
2015-04-29 19:07 ` Jan Kiszka
2015-04-29 22:21 ` Bandan Das
2015-04-30 5:40 ` Jan Kiszka
2015-04-30 16:50 ` Bandan Das
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