linux-kernel - Re: [PATCH] KVM: IOAPIC: only access APIC registers one dword at a time

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Message-ID: <4C2D95EA.6080209@np.css.fujitsu.com>
Date:	Fri, 02 Jul 2010 16:31:54 +0900
From:	Jin Dongming <jin.dongming@...css.fujitsu.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: IOAPIC: only access APIC registers one dword at
 a time

Hi, Xiao Guangrong

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.

I could understand what you described on the above, but I don't think it is 
the best method to fix it. Could you consider a nice one?

>
> So, this patch removes other width access
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...fujitsu.com>
> ---
>  virt/kvm/ioapic.c |   10 +++-------
>  1 files changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/virt/kvm/ioapic.c b/virt/kvm/ioapic.c
> index 1149c60..f96816f 100644
> --- a/virt/kvm/ioapic.c
> +++ b/virt/kvm/ioapic.c
> @@ -306,14 +306,10 @@ static int ioapic_mmio_read(struct kvm_io_device *this, gpa_t addr, int len,
>  	spin_unlock(&ioapic->lock);
>  
>  	switch (len) {
> -	case 8:
> -		*(u64 *) val = result;
> -		break;
> -	case 1:
> -	case 2:
>  	case 4:
> -		memcpy(val, (char *)&result, len);

Here the parameter len is not used for reading data from ioapic register, before switch case
the value of ioapic register has been read by ioapic_read_indirect().


> +		*(u32 *) val = result;
>  		break;
> +
>  	default:
>  		printk(KERN_WARNING "ioapic: wrong length %d\n", len);

And I think here is not good to print warning message. Maybe it is better to do
such kind of checking at the first of this function before ioapic_read_indirect().

>  	}
> @@ -332,7 +328,7 @@ static int ioapic_mmio_write(struct kvm_io_device *this, gpa_t addr, int len,
>  		     (void*)addr, len, val);
>  	ASSERT(!(addr & 0xf));	/* check alignment */
>  
> -	if (len == 4 || len == 8)
> +	if (len == 4)
>  		data = *(u32 *) val;
>  	else {
>  		printk(KERN_WARNING "ioapic: Unsupported size %d\n", len);

So I hope you could read the source code again. I think you can use a better method to fix it.

Best Regards,
Jin Dongming

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