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PCIe MRRS: Max Read Request Size: Capable of bigger size than advertised.

Making some tests with an FPGA, I found out the Intel 8th/9th gen CPUs are capable of 4KB read request size even though lspci shows 512B.

DevCtl:	Report errors: Correctable+ Non-Fatal+ Fatal+ Unsupported+
	RlxdOrd+ ExtTag+ PhantFunc- AuxPwr- NoSnoop+ FLReset-
	MaxPayload 256 bytes, MaxReadReq 512 bytes

Sending a MemRd TLP requesting 4096B (1024DWORDs) results in the reception of 16x  256B (MPS) TLPs.

* Why is that possible? Can I reliably use that result at least for that particular CPU? (i5-9600K)

* Is it a per CPU basis?

* The datasheet doesn't mention any maximum value: https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/8th-gen-core-family-datashee...

Thank you,

Alexis

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