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Date:	Sat, 03 Jul 2010 13:11:13 +0300
From:	Avi Kivity <>
To:	Xiao Guangrong <>
CC:	Marcelo Tosatti <>,
	LKML <>,
	KVM list <>,
	Jin Dongming <>
Subject: Re: [PATCH v2] KVM: IOAPIC: only access APIC registers one dword
 at a time

On 07/02/2010 11:00 AM, Xiao Guangrong wrote:
> The IOAPIC spec says:
> When accessing these registers, accesses must be done one dword at a time.
> For example, software should never access byte 2 from the Data register before
> accessing bytes 0 and 1. The hardware will not attempt to recover from a bad
> programming model in this case.
> So, this patch removes other width access

The ioapic code also implements the ia64 iosapic.  I'm guessing that 
does support 64-bit accesses.  Please check the iosapic documentation.

There might be guests that use incorrect access despite the 
documentation; if real hardware supports it, it should work.  So we need 
to start with just a warning, and allow the access.  Later we can drop 
the invalid access.

> @@ -288,6 +288,11 @@ static int ioapic_mmio_read(struct kvm_io_device *this, gpa_t addr, int len,
>   	ioapic_debug("addr %lx\n", (unsigned long)addr);
>   	ASSERT(!(addr&  0xf));	/* check alignment */
> +	if (len != 4) {
> +		printk(KERN_WARNING "ioapic: wrong length %d\n", len);
> +		return 0;
> +	}
> +

Guest triggered, so needs to be rate limited.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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