From: Wang, Wei W wei.w.wang@intel.com Sent: Wednesday, December 29, 2021 10:58 AM
Reuse KVM_SET_XSAVE for both old/new formats by reimplementing it to do properly-sized memdup_user() based on the guest fpu container.
I'm confused, the first sentence says KVM_SET_XSAVE isn't suitable, the second says it can be reused with minimal effort.
Probably "doesn't support" sounds better than "isn't suitable" above. But plan to reword a bit:
With KVM_CAP_XSAVE, userspace uses a hardcoded 4KB buffer to get/set xstate data from/to KVM. This doesn't work when dynamic features (e.g. AMX) are used by the guest, as KVM uses a full expanded xstate buffer for the guest fpu emulation, which is larger than 4KB.
Add KVM_CAP_XSAVE2, and userspace gets the required xstate buffer size from KVM via KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2). KVM_SET_XSAVE is extended with the support to work with larger xstate data size passed from userspace. KVM_GET_XSAVE2 is preferred to extending KVM_GET_XSAVE to work with large buffer size for backward- compatible considerations. (Link: https://lkml.org/lkml/2021/12/15/510)
Also, update the api doc with the new KVM_GET_XSAVE2 ioctl.
Revised to:
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With KVM_CAP_XSAVE, userspace uses a hardcoded 4KB buffer to get/set xstate data from/to KVM. This doesn't work when dynamic xfeatures (e.g. AMX) are exposed to the guest as they require a larger buffer size.
Introduce a new capability (KVM_CAP_XSAVE2). Userspace VMM gets the required xstate buffer size via KVM_CHECK_EXTENSION(KVM_CAP_XSAVE2). KVM_SET_XSAVE is extended to work with both legacy and new capabilities by doing properly-sized memdup_user() based on the guest fpu container. KVM_GET_XSAVE is kept for backward-compatible reason. Instead, KVM_GET_XSAVE2 is introduced under KVM_CAP_XSAVE2 as the preferred interface for getting xstate buffer (4KB or larger size) from KVM. (Link: https://lkml.org/lkml/2021/12/15/510)
Also, update the api doc with the new KVM_GET_XSAVE2 ioctl
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Thanks Kevin