linux-kernel - Re: [PATCH v1 3/5] KVM: selftests: Dirty host pages in dirty_log

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Date:   Mon, 22 Aug 2022 16:29:52 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Andrew Jones <andrew.jones@...ux.dev>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        peterx@...hat.com, pbonzini@...hat.com, corbet@....net,
        maz@...nel.org, james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, oliver.upton@...ux.dev,
        catalin.marinas@....com, will@...nel.org, shuah@...nel.org,
        seanjc@...gle.com, drjones@...hat.com, dmatlack@...gle.com,
        bgardon@...gle.com, ricarkol@...gle.com, zhenyzha@...hat.com,
        shan.gavin@...il.com
Subject: Re: [PATCH v1 3/5] KVM: selftests: Dirty host pages in dirty_log_test

Hi Drew,

On 8/19/22 3:28 PM, Andrew Jones wrote:
> On Fri, Aug 19, 2022 at 08:55:59AM +0800, Gavin Shan wrote:
>> It's assumed that 1024 host pages, instead of guest pages, are dirtied
>> in each iteration in guest_code(). The current implementation misses
>> the case of mismatched page sizes in host and guest. For example,
>> ARM64 could have 64KB page size in guest, but 4KB page size in host.
>> (TEST_PAGES_PER_LOOP / 16), instead of TEST_PAGES_PER_LOOP, host pages
>> are dirtied in every iteration.
>>
>> Fix the issue by touching all sub-pages when we have mismatched
>> page sizes in host and guest.
> 
> I'll let the dirty-log test authors decide what's best to do for this
> test, but I'd think we should let the guest continue dirtying its
> pages without knowledge of the host pages. Then, adjust the host test
> code to assert all sub-pages, other than the ones it expects the guest
> to have written, remain untouched.
> 

I don't think what is clarified in the change log is correct. The current
implementation already had the logic to handle the mismatched page sizes
in vm_dirty_log_verify() where 'step' is used for it by fetching value
from vm_num_host_pages(mode, 1). Please ignore this patch for now, as
explained below.

The issue I have is the 'dirty_log_test' hangs when I have 4KB host page size
and 64KB guest page size. It seems the vcpu doesn't exit due to full ring
buffer state or kick-off. I will have more investigations to figure out the
root cause.

# ./dirty_log_test -M dirty-ring -m 7
Setting log mode to: 'dirty-ring'
Test iterations: 32, interval: 10 (ms)
Testing guest mode: PA-bits:40,  VA-bits:48, 64K pages
guest physical test memory offset: 0xffbffc0000
vcpu stops because vcpu is kicked out...
Notifying vcpu to continue
vcpu continues now.
Iteration 1 collected 1903 pages
<no more output>

'dirty_lot_test' works well when both host and guest have 4KB page size.

# ./dirty_log_test -M dirty-ring -m 5
Setting log mode to: 'dirty-ring'
Test iterations: 32, interval: 10 (ms)
Testing guest mode: PA-bits:40,  VA-bits:48,  4K pages
guest physical test memory offset: 0xffbfffc000
vcpu stops because vcpu is kicked out...
Notifying vcpu to continue
vcpu continues now.
   :
Dirtied 1006592 pages
Total bits checked: dirty (1020487), clear (7106070), track_next (974104)

Thanks,
Gavin