linux-kernel - Re: [RFC PATCH 3/6] KVM: guest_memfd, x86: MEMORY

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Date:   Wed, 13 Sep 2023 10:37:24 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     isaku.yamahata@...el.com
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        isaku.yamahata@...il.com, Michael Roth <michael.roth@....com>,
        Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
        Sagi Shahar <sagis@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Kai Huang <kai.huang@...el.com>,
        Zhi Wang <zhi.wang.linux@...il.com>, chen.bo@...el.com,
        linux-coco@...ts.linux.dev,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yuan Yao <yuan.yao@...ux.intel.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Xu Yilun <yilun.xu@...el.com>,
        Quentin Perret <qperret@...gle.com>, wei.w.wang@...el.com,
        Fuad Tabba <tabba@...gle.com>
Subject: Re: [RFC PATCH 3/6] KVM: guest_memfd, x86: MEMORY_FAULT exit with hw
 poisoned page

On Wed, Sep 13, 2023, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> When resolving kvm page fault and hwpoisoned page is given, KVM exit
> with HWPOISONED flag so that user space VMM, e.g. qemu, handle it.
> 
> - Add a new flag POISON to KVM_EXIT_MEMORY_FAULT to indicate the page is
>   poisoned.
> - Make kvm_gmem_get_pfn() return hwpoison state by -EHWPOISON when the
>   folio is hw-poisoned.
> - When page is hw-poisoned on faulting in private gmem, return
>   KVM_EXIT_MEMORY_FAULT with HWPOISONED flag.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---

...

> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index eb900344a054..48329cb44415 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -527,7 +527,8 @@ struct kvm_run {
>  		} notify;
>  		/* KVM_EXIT_MEMORY_FAULT */
>  		struct {
> -#define KVM_MEMORY_EXIT_FLAG_PRIVATE	(1ULL << 3)
> +#define KVM_MEMORY_EXIT_FLAG_PRIVATE	BIT_ULL(3)
> +#define KVM_MEMORY_EXIT_FLAG_HWPOISON	BIT_ULL(4)

Rather than add a flag, I think we should double down on returning -1 + errno
when exiting with vcpu->run->exit_reason == KVM_EXIT_MEMORY_FAULT, as is being
proposed in Anish's series for accelerating UFFD-like behavior in KVM[*].

Then KVM can simply return -EFAULT or -EHWPOISON to communicate why KVM is
existing at a higher level, and let the kvm_run structure provide the finer
details about the access itself.  E.g. kvm_faultin_pfn_private() can simply
propagate the return value from kvm_gmem_get_pfn() without having to identify
*why* kvm_gmem_get_pfn() failed.

static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
				   struct kvm_page_fault *fault)
{
	int max_order, r;

	if (!kvm_slot_can_be_private(fault->slot)) {
		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
		return -EFAULT;
	}

	r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
			     &max_order);
	if (r) {
		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
		return r;
	}

	...
}

[*] https://lore.kernel.org/all/20230908222905.1321305-5-amoorthy@google.com