linux-kernel - Re: [PATCH v2 5/6] KVM: x86: Keep a per-VM MTRR state

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Date:   Tue, 23 May 2023 14:21:30 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Robert Hoo <robert.hoo.linux@...il.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <pbonzini@...hat.com>, <seanjc@...gle.com>
Subject: Re: [PATCH v2 5/6] KVM: x86: Keep a per-VM MTRR state

On Sun, May 21, 2023 at 11:44:36AM +0800, Robert Hoo wrote:
> On 5/9/2023 9:53 PM, Yan Zhao wrote:
> > Keep a per-VM MTRR state and point it to the MTRR state of vCPU 0.
> > 
> > This is a preparation patch for KVM to reference a per-VM guest MTRR
> > to decide memory type of EPT leaf entries when noncoherent DMA is present.
> > 
> > Though each vCPU has its own MTRR state, MTRR states should be
> > consistent across each VM, which is demanded as in Intel's SDM
> > "In a multiprocessor system using a processor in the P6 family or a more
> > recent family, each processor MUST use the identical MTRR memory map so
> > that software will have a consistent view of memory."
> > 
> > Therefore, when memory type of EPT leaf entry needs to honor guest MTRR,
> > a per-VM version of guest MTRR can be referenced.
> > 
> > Each vCPU still has its own MTRR state field to keep guest rdmsr()
> > returning the right value when there's lag of MTRR update for each vCPU.
> > 
> Can we get rid of per-vCPU MTRR state copies and just have this per-VM state
> only? therefore can simplify implementation and avoid hazard of
> inconsistency among per-VPU MTRR states.
> 
> I see in SDM, it notes:
> "In multiple processor systems, the operating system must maintain MTRR
> consistency between all the processors in the system (that is, all
> processors must use the same MTRR values). The P6 and more recent processor
> families provide no hardware support for maintaining this consistency."
> 
> leaving each vCPU's MTRR is just to fully mimic HW?
>
Yes, leaving each vCPU's MTRR to mimic HW.

As also suggested in SDM, the guest OS manipulates MTRRs in this way:

for each online CPUs {
	disable MTRR
	update fixed/var MTRR ranges
	enable MTRR
}
Guest OS needs to access memory only after this full pattern.

So, I think there should not have "hazard of inconsistency among per-VPU MTRR
states".

I want to have per-VM MTRR state is because I want to reduce unnessary EPT
zap, which costs quite a lot cpu cycles even when the EPT is empty.

In this patch, per-VM MTRR pointer is used to point to vCPU 0's MTRR state,
so that it can save some memory to keep the MTRR state.
But I found out that it would only work when vCPU 0 (boot processor) is
always online (which is not true for x86 under some configration).

I'll try to find out lowest online vCPU and keep a per-VM copy of MTRR state
in next version.

Thanks!