[PATCH] KVM: allow NULL writable argument to __kvm_faultin_pfn

[Christian Zigotzky <chzigotzky@xenosoft.de>]



> On 06 January 2025 at 07:57 pm, Sean Christopherson <seanjc@google.com> 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@xenosoft.de>

Thanks