[RFC PATCH v2 4/8] KVM: arm64: Set DBM for previously writeable pages

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Catalin Marinas [mailto:catalin.marinas at arm.com]
> Sent: 22 September 2023 16:40
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi at huawei.com>
> Cc: kvmarm at lists.linux.dev; kvm at vger.kernel.org;
> linux-arm-kernel at lists.infradead.org; maz at kernel.org; will at kernel.org;
> oliver.upton at linux.dev; james.morse at arm.com; suzuki.poulose at arm.com;
> yuzenghui <yuzenghui at huawei.com>; zhukeqian
> <zhukeqian1 at huawei.com>; Jonathan Cameron
> <jonathan.cameron at huawei.com>; Linuxarm <linuxarm at huawei.com>
> Subject: Re: [RFC PATCH v2 4/8] KVM: arm64: Set DBM for previously
> writeable pages
> 
> On Fri, Aug 25, 2023 at 10:35:24AM +0100, Shameer Kolothum wrote:
> > We only set DBM if the page is writeable (S2AP[1] == 1). But once
> migration
> > starts, CLEAR_LOG path will write protect the pages (S2AP[1] = 0) and
> there
> > isn't an easy way to differentiate the writeable pages that gets write
> > protected from read-only pages as we only have S2AP[1] bit to check.
> 
> Don't we have enough bits without an additional one?
> 
> writeable: DBM == 1 || S2AP[1] == 1
>   clean: S2AP[1] == 0
>   dirty: S2AP[1] == 1 (irrespective of DBM)
> 
> read-only: DBM == 0 && S2AP[1] == 0
> 
> For S1 we use a software dirty bit as well to track read-only dirty
> mappings but I don't think it is necessary for S2 since KVM unmaps the
> PTE when changing the VMM permission.
> 

We don't set the DBM for all the memory. In order to reduce the overhead
associated with scanning PTEs, this series sets the DBM for the nearby pages
on page fault during the migration phase.

See patch #8,
  user_mem_abort()
     kvm_arm_enable_nearby_hwdbm()

But once migration starts, on CLEAR_LOG path,
   kvm_arch_mmu_enable_log_dirty_pt_masked()
    stage2_wp_range()  --> set the page read only
    kvm_mmu_split_huge_pages() --> split huge pages and pages are read only.

This in effect means there are no writeable-clean near-by pages to set the DBM on
kvm_arm_enable_nearby_hwdbm().

To identify the pages that can be set DBM, we provide a hint to
stage2_wp_range( ) --> kvm_pgtable_stage2_wrprotect() table walker and make
use of a new software bit to mark the PTE as writeable-clean.

Hope, I am clear.

Thanks,
Shameer