linux-kernel - Re: [PATCH v4 3/4] KVM: x86: Use kvzalloc() to allocate VM struct

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Message-ID: <aea0cc02-3b18-4c7e-9108-ab5e923051bf@intel.com>
Date: Wed, 28 May 2025 16:35:52 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Vipin Sharma <vipinsh@...gle.com>, James Houghton <jthoughton@...gle.com>
Subject: Re: [PATCH v4 3/4] KVM: x86: Use kvzalloc() to allocate VM struct

On 5/23/2025 8:11 AM, Sean Christopherson wrote:
> Allocate VM structs via kvzalloc(), i.e. try to use a contiguous physical
> allocation before falling back to __vmalloc(), to avoid the overhead of
> establishing the virtual mappings.  For non-debug builds, The SVM and VMX
> (and TDX) structures are now just below 7000 bytes in the worst case
> scenario (see below), i.e. are order-1 allocations, and will likely remain
> that way for quite some time.
> 
> Add compile-time assertions in vendor code to ensure the size of the
> structures, sans the memslos hash tables, are order-0 allocations, i.e.

s/memslos/memslots

> are less than 4KiB.  There's nothing fundamentally wrong with a larger
> kvm_{svm,vmx,tdx} size, but given that the size of the structure (without
> the memslots hash tables) is below 2KiB after 18+ years of existence,
> more than doubling the size would be quite notable.
> 
> Add sanity checks on the memslot hash table sizes, partly to ensure they
> aren't resized without accounting for the impact on VM structure size, and
> partly to document that the majority of the size of VM structures comes
> from the memslots.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>