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From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Bandan Das <bsd@redhat.com>
Cc: Hardik H Bagdi <hbagdi1@binghamton.edu>,
	kvm@vger.kernel.org, Igor Mammedov <imammedo@redhat.com>
Subject: Re: KVM_MAX_VCPU hard limit of 255 on x86
Date: Mon, 4 Apr 2016 17:17:19 +0200	[thread overview]
Message-ID: <20160404151719.GA21537@potion.brq.redhat.com> (raw)
In-Reply-To: <jpgd1q9q9j8.fsf@linux.bootlegged.copy>

2016-04-01 17:55-0400, Bandan Das:
> Hardik H Bagdi <hbagdi1@binghamton.edu> writes:
>> Hello,
>>
>> For research, I am experimenting with increasing the number of VCPUs
>> in the guest OS.
>> I can increase the number of VCPUs till 255 in the guest OS but more
>> than that results in the following error-
> 
> Radim (Cced) might have more info but the short answer is that you
> can't just simply increase the number. For >255, you need x2apic
> with interrupt remapping which is still missing. 

Yes, interrupt remapping is being developed for the split irqchip.
You could hack around that (we chose not to) so another blocker is that
QEMU cannot handle more than 255 VCPUs, mainly because of its AML
generator.  Igor (Cc'd) knows more about the status of ACPI.

(And firmware doesn't implement x2APIC, and other minor problems ...)

All pieces should be in place within half a year, though.
The next hard limit is ~2^20 or ~2^32.

  reply	other threads:[~2016-04-04 15:17 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-04-01 21:19 KVM_MAX_VCPU hard limit of 255 on x86 Hardik H Bagdi
2016-04-01 21:55 ` Bandan Das
2016-04-04 15:17   ` Radim Krčmář [this message]
2016-04-04 19:14     ` Igor Mammedov
2016-04-04 20:34       ` Radim Krčmář
2016-04-05 11:20         ` Igor Mammedov
2016-04-05 14:01           ` Radim Krčmář

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