Re: [RFC PATCH v2 14/23] KVM: Allow 2048-bit register access via ioctl interface

From: "Alex Bennée" <alex.bennee@linaro.org>
To: Dave Martin <Dave.Martin@arm.com>
Cc: Okamoto Takayuki <tokamoto@jp.fujitsu.com>,
	Christoffer Dall <cdall@kernel.org>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH v2 14/23] KVM: Allow 2048-bit register access via ioctl interface
Date: Mon, 19 Nov 2018 16:48:36 +0000	[thread overview]
Message-ID: <87r2fhhv2z.fsf@linaro.org> (raw)
In-Reply-To: <1538141967-15375-15-git-send-email-Dave.Martin@arm.com>

Dave Martin <Dave.Martin@arm.com> writes:

> The Arm SVE architecture defines registers that are up to 2048 bits
> in size (with some possibility of further future expansion).
>
> In order to avoid the need for an excessively large number of
> ioctls when saving and restoring a vcpu's registers, this patch
> adds a #define to make support for individual 2048-bit registers
> through the KVM_{GET,SET}_ONE_REG ioctl interface official.  This
> will allow each SVE register to be accessed in a single call.
>
> There are sufficient spare bits in the register id size field for
> this change, so there is no ABI impact providing that
> KVM_GET_REG_LIST does not enumerate any 2048-bit register unless
> userspace explicitly opts in to the relevant architecture-specific
> features.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
>  include/uapi/linux/kvm.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 251be35..7c3c5cc 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1110,6 +1110,7 @@ struct kvm_dirty_tlb {
>  #define KVM_REG_SIZE_U256	0x0050000000000000ULL
>  #define KVM_REG_SIZE_U512	0x0060000000000000ULL
>  #define KVM_REG_SIZE_U1024	0x0070000000000000ULL
> +#define KVM_REG_SIZE_U2048	0x0080000000000000ULL

Yeah OK I guess - but it does make me question if KVM_REG_SIZE_MASK is
part of the ABI because although we have space for another few bits that
is the last one without changing the mask.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

>
>  struct kvm_reg_list {
>  	__u64 n; /* number of regs */

--
Alex Bennée
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