linux-kernel - Re: [RFC v2 03/17] x86/apic: Populate .read()/.write() callbacks of Secure AVIC driver

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Message-ID: <87plia33mu.ffs@tglx>
Date: Fri, 21 Mar 2025 14:38:33 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Neeraj Upadhyay <Neeraj.Upadhyay@....com>, linux-kernel@...r.kernel.org
Cc: bp@...en8.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
 Thomas.Lendacky@....com, nikunj@....com, Santosh.Shukla@....com,
 Vasant.Hegde@....com, Suravee.Suthikulpanit@....com, David.Kaplan@....com,
 x86@...nel.org, hpa@...or.com, peterz@...radead.org, seanjc@...gle.com,
 pbonzini@...hat.com, kvm@...r.kernel.org, kirill.shutemov@...ux.intel.com,
 huibo.wang@....com, naveen.rao@....com
Subject: Re: [RFC v2 03/17] x86/apic: Populate .read()/.write() callbacks of
 Secure AVIC driver

On Wed, Feb 26 2025 at 14:35, Neeraj Upadhyay wrote:

> +static inline u32 get_reg(char *page, int reg)
> +{
> +	return READ_ONCE(*((u32 *)(page + reg)));

This type casting is disgusting. First you cast the void pointer of the
per CPU backing page implicitely into a signed character pointer and
then cast that to a u32 pointer. Seriously?

struct apic_page {
	union {
		u32	regs[NR_APIC_REGS];
                u8	bytes[PAGE_SIZE];
	};
};                

and the per CPU allocation of apic_page makes this:

static __always_inline u32 get_reg(unsigned int offset)
{
        return READ_ONCE(this_cpu_ptr(apic_page)->regs[offset >> 2]));
}

which avoids the whole pointer indirection of your backing page construct.

Thanks,

        tglx