linux-kernel - Re: [PATCH] kvm: x86: reduce rtc 0x70 access vm-exit time

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Date:   Wed, 2 Aug 2017 12:06:37 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     peng.hao2@....com.cn
Cc:     rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: x86: reduce rtc 0x70 access vm-exit time

On 02/08/2017 11:02, peng.hao2@....com.cn wrote:
> 
>> On 02/08/2017 10:45, peng.hao2@....com.cn wrote:
>> >>On 02/08/2017 17:24, Peng Hao wrote:
>> > coalesced MMIO is async and io port should be sync .  I don't need to
>> > coalese io port access.
> 
>> Coalesced MMIO access is resolved on the next read from a coalesced MMIO
>> address or the next write to a non-coalescecd MMIO address.  So it would
>> work perfectly for port 0x70.  You would have to modify QEMU to split
>> the RTC I/O ports into two MemoryRegions.
>
> ok , it sounds simpler. I will try it.

My suggestion is to repurpose the "pad" field of "struct
kvm_coalesced_mmio_zone" to be 0 for memory and -1 for I/O.  It should
be enough for an RFC.

Later we can look into how to handle backwards compatibility.

Thanks,

Paolo