> On 06 January 2025 at 07:57 pm, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Jan 01, 2025, Paolo Bonzini wrote:
>> kvm_follow_pfn() is able to work with NULL in the .map_writable field
>> of the homonymous struct. But __kvm_faultin_pfn() rejects the combo
>> despite KVM for e500 trying to use it. Indeed .map_writable is not
>> particularly useful if the flags include FOLL_WRITE and readonly
>> guest memory is not supported, so add support to __kvm_faultin_pfn()
>> for this case.
>
> I would prefer to keep the sanity check to minimize the risk of a page fault
> handler not supporting opportunistic write mappings. e500 is definitely the
> odd one out here.
>
> What about adding a dedicated wrapper for getting a writable PFN? E.g. (untested)
>
> ---
> arch/powerpc/kvm/e500_mmu_host.c | 2 +-
> arch/x86/kvm/vmx/vmx.c | 3 +--
> include/linux/kvm_host.h | 8 ++++++++
> 3 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kvm/e500_mmu_host.c b/arch/powerpc/kvm/e500_mmu_host.c
> index e5a145b578a4..2251bb30b8ec 100644
> --- a/arch/powerpc/kvm/e500_mmu_host.c
> +++ b/arch/powerpc/kvm/e500_mmu_host.c
> @@ -444,7 +444,7 @@ static inline int kvmppc_e500_shadow_map(struct kvmppc_vcpu_e500 *vcpu_e500,
>
> if (likely(!pfnmap)) {
> tsize_pages = 1UL << (tsize + 10 - PAGE_SHIFT);
> - pfn = __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, NULL, &page);
> + pfn = kvm_faultin_writable_pfn(slot, gfn, &page);
> if (is_error_noslot_pfn(pfn)) {
> if (printk_ratelimit())
> pr_err("%s: real page not found for gfn %lx\n",
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 893366e53732..7012b583f2e8 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6800,7 +6800,6 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
> struct page *refcounted_page;
> unsigned long mmu_seq;
> kvm_pfn_t pfn;
> - bool writable;
>
> /* Defer reload until vmcs01 is the current VMCS. */
> if (is_guest_mode(vcpu)) {
> @@ -6836,7 +6835,7 @@ void vmx_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
> * controls the APIC-access page memslot, and only deletes the memslot
> * if APICv is permanently inhibited, i.e. the memslot won't reappear.
> */
> - pfn = __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, &writable, &refcounted_page);
> + pfn = kvm_faultin_writable_pfn(slot, gfn, &refcounted_page);
> if (is_error_noslot_pfn(pfn))
> return;
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index c788d0bd952a..b0af7c7f99da 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1287,6 +1287,14 @@ static inline kvm_pfn_t kvm_faultin_pfn(struct kvm_vcpu *vcpu, gfn_t gfn,
> write ? FOLL_WRITE : 0, writable, refcounted_page);
> }
>
> +static inline kvm_pfn_t kvm_faultin_writable_pfn(const struct kvm_memory_slot *slot,
> + gfn_t gfn, struct page **refcounted_page)
> +{
> + bool writable;
> +
> + return __kvm_faultin_pfn(slot, gfn, FOLL_WRITE, &writable, refcounted_page);
> +}
> +
> int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
> int len);
> int kvm_read_guest(struct kvm *kvm, gpa_t gpa, void *data, unsigned long len);
>
> base-commit: 2c3412e999738bfd60859c493ff47f5c268814a3
> --
This patch works. Tested-by: Christian Zigotzky <chzigotzky@xxxxxxxxxxx>
Thanks
