linux-kernel - [PATCH 0/5] KVM: e500: map readonly host pages for read, and cleanup

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Message-ID: <20250109133817.314401-1-pbonzini@redhat.com>
Date: Thu,  9 Jan 2025 14:38:12 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: oliver.upton@...ux.dev,
	Will Deacon <will@...nel.org>,
	Anup Patel <apatel@...tanamicro.com>,
	Andrew Jones <ajones@...tanamicro.com>,
	seanjc@...gle.com,
	linuxppc-dev@...ts.ozlabs.org,
	regressions@...ts.linux.dev
Subject: [PATCH 0/5] KVM: e500: map readonly host pages for read, and cleanup

[Oliver/Will/Anup/Andrew, you're Cc'd because of an observation below
 on VM_PFNMAP mappings. - Paolo]

The new __kvm_faultin_pfn() function is upset by the fact that e500
KVM ignores host page permissions - __kvm_faultin requires a "writable"
outgoing argument, but e500 KVM is passing NULL.

While a simple fix would be possible that simply allows writable to
be NULL, it is quite ugly to have e500 KVM ignore completely the host
permissions and map readonly host pages as guest-writable.  A more
complete fix is present in the second to fourth patches (the first is
an independent bugfix, Cc'd to stable).

The last one removes the VMA-based attempts at building huge shadow TLB
entries, in favor of using a PTE lookup similar to what is done for x86.
This special casing of VM_PFNMAP does not work well with remap_pfn_range()
as it assumes that VM_PFNMAP areas are contiguous.  Note that the same
incorrect logic is there in ARM's get_vma_page_shift() and RISC-V's
kvm_riscv_gstage_ioremap().

Fortunately, for e500 most of the code is already there; it just has to
be changed to compute the range from find_linux_pte()'s output rather
than find_vma().  The new code works for both VM_PFNMAP and hugetlb
mappings, so the latter is removed.

If this does not work out I'll go for something like
https://lore.kernel.org/kvm/Z3wnsQQ67GBf1Vsb@google.com/, but
with the helper in arch/powerpc/kvm/e500_mmu_host.c.

The series is compile-tested only.  Christian, please test
this as we do not have e500 hardware readily availabe.

Thanks,

Paolo

Supersedes: <20250101064928.389504-1-pbonzini@...hat.com>

Paolo Bonzini (5):
  KVM: e500: retry if no memslot is found
  KVM: e500: use shadow TLB entry as witness for writability
  KVM: e500: track host-writability of pages
  KVM: e500: map readonly host pages for read
  KVM: e500: perform hugepage check after looking up the PFN

 arch/powerpc/kvm/e500.h          |   2 +
 arch/powerpc/kvm/e500_mmu_host.c | 202 +++++++++++++------------------
 2 files changed, 89 insertions(+), 115 deletions(-)

-- 
2.47.1