From: Paolo Bonzini <pbonzini@redhat.com>
To: Alexander Graf <graf@amazon.com>,
Siddharth Chandrasekaran <sidcha@amazon.de>,
Sean Christopherson <seanjc@google.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Cc: Evgeny Iakovlev <eyakovl@amazon.de>,
Liran Alon <liran@amazon.com>,
Ioannis Aslanidis <iaslan@amazon.de>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] KVM: hyper-v: Add new exit reason HYPERV_OVERLAY
Date: Fri, 23 Apr 2021 12:15:01 +0200 [thread overview]
Message-ID: <45888d26-89d2-dba6-41cb-de2d58cd5345@redhat.com> (raw)
In-Reply-To: <ded8db53-0e58-654a-fff2-de536bcbc961@amazon.com>
On 23/04/21 11:58, Alexander Graf wrote:
>> In theory userspace doesn't know how KVM wishes to implement the
>> hypercall page, especially if Xen hypercalls are enabled as well.
>
> I'm not sure I agree with that sentiment :). User space is the one that
> sets the xen compat mode. All we need to do is declare the ORing as part
> of the KVM ABI. Which we effectively are doing already, because it's
> part of the ABI to the guest, no?
Good point. But it may change in the future based on KVM_ENABLE_CAP or
whatever, and duplicating code between userspace and kernel is ugly. We
already have too many unwritten conventions around CPUID, MSRs, get/set
state ioctls, etc.
That said, this definitely tilts the balance against adding an ioctl to
write the hypercall page contents. Userspace can either use the
KVM_SET_MSR or assemble it on its own, and one of the two should be okay.
Paolo
>>
>> But userspace has two plausible ways to get the page contents:
>>
>> 1) add a ioctl to write the hypercall page contents to an arbitrary
>> userspace address
>>
>> 2) after userspace updates the memslots to add the overlay page at the
>> right place, use KVM_SET_MSR from userspace (which won't be filtered
>> because it's host initiated)
>>
>> The second has the advantage of not needing any new code at all, but
>> it's a bit more ugly.
>
> The more of all of that hyper-v code we can have live in user space, the
> happier I am :).
next prev parent reply other threads:[~2021-04-23 10:15 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-23 9:03 [PATCH] KVM: hyper-v: Add new exit reason HYPERV_OVERLAY Siddharth Chandrasekaran
2021-04-23 9:24 ` Alexander Graf
2021-04-23 9:42 ` Siddharth Chandrasekaran
2021-04-23 9:50 ` Paolo Bonzini
2021-04-23 9:58 ` Alexander Graf
2021-04-23 10:15 ` Paolo Bonzini [this message]
2021-04-23 10:18 ` Alexander Graf
2021-04-26 21:41 ` Siddharth Chandrasekaran
2021-04-23 9:36 ` Vitaly Kuznetsov
2021-04-23 10:05 ` Siddharth Chandrasekaran
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