Re: [PATCH v2] KVM: IOAPIC: only access APIC registers one dword at a time

Xiao Guangrong
2010-07-05T03:50:58Z

Avi Kivity wrote:
> 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.
>

The iosapic also using 32-bit to access registers:

All registers are accessed using 32-bit uncacheable loads and stores to a reserved memory location
in system memory. This implies that to modify a field (e.g., a bit or a byte) in any register, the
whole 32-bit register must be read, the field modified, and the 32 bits written back. Partial register
access, or non-aligned register access, are implementation-defined by the I/O xAPIC and will not
be compatible across different implementations. Also, registers that are described as 64 bits wide
are accessed as multiple independent 32-bit registers.

[ From << Intel® Itanium® Processor Family Interrupt Architecture Guide >>, P2-6 ]

> 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.

If the OS contravene the spec, i thinks it's the OS's bug, also, i have tested some versions
windows/linux guests, it's no broken, can we directly drop the other wide 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.

Yeah, will using printk_ratelimit cooperate with it.